OBSERVARE

Universidade Autónoma de Lisboa

e-ISSN: 1647-7251

VOL. 17, Nº. 1

May 2026

100

INDEPENDENT WOMEN ON BOARDS OF DIRECTORS: INTRODUCING THE

“SANDWICH EFFECT” CONCEPT

ANA QUARESMA

aquaresma@autonoma.pt

She holds a PhD in Economics and is an Assistant Professor at Universidade Autónoma de Lisboa

(Portugal), focusing on corporate governance research. She previously worked for nearly three

decades in the banking sector, including leadership roles. https://orcid.org/0000-0001-8502-

4033

SANDRA RIBEIRO

sribeiro@autonoma.pt

Sandra Ribeiro holds a PhD in Economics and has taught at Universidade Autónoma de Lisboa

(Portugal) since 1999, with publications in international journals and conferences. Her research

areas include International Economics and Macroeconomics. https://orcid.org/0000-0002-8995-

5684

Abstract

This article introduces the Sandwich Effect, a new conceptual model that explains how

independent women directors operate at the intersection of regulatory governance

requirements and market pressures for inclusion, legitimacy and transparency. Drawing on

Agency, Institutional and Resource Dependence theories, the model highlights the dual role

played by independent female directors as agents of compliance and as carriers of market-

signaling effects. To illustrate its relevance, the article presents an exploratory analysis of

PSI‑listed companies, examining gender composition, independence, adherence to

governance recommendations, and market valuation indicators. While the empirical patterns

show no significant association between gender diversity and ROE, they reveal a strong

positive relationship between the presence of independent women, compliance with

governance best practices, and market valuation, supporting the model’s central premise that

qualified diversity is valued by external stakeholders. The study offers a replicable theoretical

framework that enhances understanding of how board structure, independence, and gender

interact to influence governance quality and market perception.

Keywords

Gender diversity, corporate governance, boards of directors, Sandwich Effect, PSI,

independent women, gender quotas.

Resumo

Este artigo apresenta o «Efeito Sanduíche», um novo modelo conceptual que explica como as

administradoras independentes atuam na intersecção entre os requisitos regulamentares de

governação e as pressões do mercado em prol da inclusão, da legitimidade e da transparência.

Baseando-se nas teorias da Agência, Institucional e da Dependência de Recursos, o modelo

destaca o duplo papel desempenhado pelas administradoras independentes como agentes de

conformidade e como transmissoras de efeitos de sinalização do mercado. Para ilustrar a sua

relevância, o artigo apresenta uma análise exploratória de empresas cotadas na PSI,

examinando a composição de género, a independência, a adesão às recomendações de

governação e os indicadores de avaliação de mercado. Embora os padrões empíricos não

mostrem uma associação significativa entre a diversidade de género e o ROE, revelam uma

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forte relação positiva entre a presença de mulheres independentes, o cumprimento das

melhores práticas de governação e a avaliação de mercado, apoiando a premissa central do

modelo de que a diversidade qualificada é valorizada pelas partes interessadas externas. O

estudo oferece um quadro teórico replicável que melhora a compreensão de como a estrutura

do conselho de administração, a independência e o género interagem para influenciar a

qualidade da governação e a perceção do mercado.

Palavras-chave

Diversidade de género, governação corporativa, conselhos de administração, Efeito

Sanduíche, PSI, mulheres independentes, quotas de género.

How to cite this article

Quaresma, Ana & Ribeiro, Sandra (2026). Independent Women on Boards of Directors: Introducing

the “Sandwich Effect” Concept. Janus.net, e-journal of international relations, VOL. 17, Nº. 1, May

2026, pp. 100-124. https://doi.org/10.26619/1647-7251.17.1.6

Article submitted on 17 February 2026 and accepted on 1 March 2026.

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e-ISSN: 1647-7251

VOL. 17, Nº. 1

May 2026, pp. 100-124

Independent Women on Boards of Directors: Introducing the “Sandwich Effect” Concept

Ana Quaresma, Sandra Ribeiro

102

INDEPENDENT WOMEN ON BOARDS OF DIRECTORS:

INTRODUCING THE “SANDWICH EFFECT” CONCEPT

ANA QUARESMA

SANDRA RIBEIRO

Introduction

Gender diversity on boards of directors has emerged as one of the pillars of sound

corporate governance, not only for ethical reasons but also for its strategic potential

regarding sustainability, reputation, and business competitiveness (Adams & Ferreira,

2009; Terjesen et al., 2009). In Portugal, this debate gained renewed momentum with

the transposition of European Directive 2022/2381, which reinforces the national

commitment to gender equality in the decision-making bodies of listed companies. In

parallel, the Portuguese Securities Market Commission (CMVM) Corporate Governance

Code, revised in 2023, recommends that at least one-third of non-executive directors be

independent, emphasizing independence as a central element of good governance

practices. These regulatory requirements place diversity and independence at the core

of corporate and institutional agendas, making it essential to understand their impact on

firm performance and market valuation, particularly among listed companies.

This article is primarily a conceptual contribution, introducing the Sandwich Effect as a

theoretical lens to explain how independent women directors operate under simultaneous

regulatory and market pressures.

Despite the growing consensus on the importance of diversity, empirical studies continue

to report ambiguous results regarding the impact of women’s presence on boards of

directors on firms’ financial performance (Joecks et al., 2013). This ambiguity highlights

the need for more refined theoretical approaches that consider not only the institutional

context but also the internal and external dynamics of boards. The combination of gender

diversity and functional independence may enhance board oversight and foster more

balanced governance, reinforcing the need to further explore their impact on listed

companies.

In this context, the present article develops and introduces the innovative concept of the

‘Sandwich Effect,’ which positions female directors—particularly independent ones—as

agents operating between two forces: the normative requirements of Corporate

Governance best practices and market pressures for inclusion and reputation. This

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approach provides an integrated perspective on the strategic role of women on boards,

highlighting their ability to mediate between often conflicting interests.

The present study aims to:

(i) Empirically test whether the presence of women on the boards of directors

of PSI-listed companies is associated with better financial performance,

measured by Return on Equity (ROE).

(ii) Compare the impact of the percentage of independent female directors with

the overall percentage of women on boards, assessing which dimension

shows a stronger correlation with ROE.

(iii) Analyze whether compliance with Corporate Governance best practices—

particularly the inclusion of independent women on boards—is positively

associated with market valuation and female representation.

(iv) Introduce and contextualize the theoretical concept of the “Sandwich

Effect” as a framework for understanding the strategic role of women

directors in balancing external market pressures and internal regulatory

requirements.

Although the article includes an empirical illustration based on PSI-listed companies, its

primary contribution is conceptual, focusing on the development and contextualization of

the Sandwich Effect as a theoretical model

The relevance of this study lies in the integration of a quantitative analysis with an

innovative theoretical perspective, which sheds light on the challenges and opportunities

faced by female directors. By focusing on the PSI index and European policy

recommendations, the article offers an original contribution to the academic debate and

to the development of more inclusive and effective governance policies.

Collectively, this positioning underscores the article’s primary contribution: advancing a

testable conceptual model—the Sandwich Effect—that links board composition,

independence, and market perception within a clear European governance context.

Theoretical Framework

Recent research (2022–2025) reinforces that board gender diversity is increasingly

analyzed within the broader context of governance quality, risk mitigation and

stakeholder trust.

Studies such as Brahma et al. (2021), Hyun et al. (2023) and Klettner (2014) argue that

gender-diverse boards strengthen monitoring effectiveness and enhance corporate

transparency, particularly in regulated markets. New empirical findings show that

gender-diverse boards are associated with lower earnings management (Sial et al.,

2023), higher ESG disclosure quality (Adams & Ragunathan, 2023) and improved

decision-making under uncertainty (Martínez-Jiménez & Hernández-Ortega, 2024).

These recent contributions reinforce the relevance of diversity not only as a

representational metric but also as a marker of governance maturity.

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Recent evidence consolidates a shift from purely representational approaches to qualified

diversity, in which gender diversity interacts with board independence to strengthen

monitoring capacity, transparency and stakeholder confidence (Gabaldon et al., 2020;

Labelle et al., 2021). Within European markets, new contributions indicate that boards

combining female representation with independence tend to align more closely with ESG

expectations and to exhibit stronger oversight effects, consistent with the monitoring role

emphasized in prior finance and governance research (Bernile, Bhagwat, & Yonker, 2018;

Adams & Ragunathan, 2023). These developments resonate with the Sandwich Effect

proposition, in which independent women operate at the intersection of regulatory

obligations and market‑signaling dynamics (Stoeckl & Luedicke, 2024; Martínez‑Jiménez

& Hernández‑Ortega, 2024).

Beyond academic work, international organizations have documented structural drivers

and institutional conditions that enable women’s effective participation in

decision‑making. The International Labour Organization (ILO), in its Women in Business

and Management: The Business Case for Change and subsequent global leadership

diagnostics, shows that gender‑balanced leadership is associated with stronger oversight,

greater transparency and lower reputational risk, particularly when board appointments

are conducted through transparent processes that safeguard independence and reduce

tokenism (ILO, 2019; ILO, 2022). These international models align with recent European

governance developments, which place independence criteria alongside diversity targets

and are increasingly read by markets as signals of governance quality and organizational

maturity (European Commission, 2024; ESMA, 2025). Taken together, this evidence

strengthens the conceptual logic of the Sandwich Effect by indicating that diversity is

most effective when paired with independence, thereby enhancing monitoring capacity

and stakeholder confidence (García‑Sánchez, Martínez‑Ferrero, & García‑Meca, 2021;

Hyun, Park, & Kim, 2023).

Consistent with this, the European framework has intensified incentives for substantive—

not merely symbolic—compliance. The gender balance directive and updated governance

codes raise the salience of independence criteria alongside diversity targets, which the

market increasingly reads as a proxy for governance quality and organizational maturity

(European Commission, 2024; ESMA, 2025). In parallel, international labour statistics

and leadership diagnostics underline the persistence of structural bottlenecks to women’s

advancement and highlight that sustained progress depends on transparent appointment

processes and independence safeguards—conditions that reduce tokenism and amplify

real oversight (International Labour Organization, 2024; Adams & Ragunathan, 2023).

Empirically, recent studies associate gender‑diverse and independence‑balanced boards

with better audit committee effectiveness, higher disclosure quality and lower earnings

management, supporting the view that independent female directors can carry

disproportionate governance effects (Sial, Zheng, & Cherian, 2023; McGuire &

Sheridan, 2023).

Overall, the 2020–2025 literature strengthens the conceptual foundations of the

Sandwich Effect: independent women directors act simultaneously as agents of

compliance with governance norms and as carriers of market signals linked to

transparency, legitimacy and sustainability orientation (García‑Sánchez,

Martínez‑Ferrero, & García‑Meca, 2021; Hyun, Park, & Kim, 2023). Incorporating these

updated perspectives aligns the article with contemporary European and international

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developments and clarifies why independence conditions the effectiveness of gender

diversity in practice (Grosvold, Brammer, & Rayton, 2020; Dang, Bender, &

Scotto, 2021).

Classic contributions conceptualize women directors as operating between institutional

and market pressures (e.g., Bilimoria & Piderit, 2007), and this view women on boards

of directors as agents operating between two pressures: institutional demands for

diversity and market expectations for performance. This dual position enables them to

influence both the adoption of Corporate Governance best practices and the creation of

economic value. Authors such as Gabaldon et al. (2020) reinforce this view, arguing that

women on boards act as mediators between institutional pressures and market demands,

promoting more ethical and inclusive governance practices. Similarly, Labelle et al.

(2021) emphasize this perspective, highlighting the role of women as mediators between

social responsibility and performance.

− Presence of Women on Boards of Directors and Their Positive Association

with ROE

− Gender diversity on boards has been associated with greater oversight

effectiveness, innovation, and ethical sensitivity. Studies such as García-

Izquierdo et al. (2021) show that the presence of women can enhance

financial performance, particularly in contexts with strong institutional

pressure. Nadeem et al. (2020) and Dang et al. (2021) further reinforce

that gender diversity can improve financial outcomes, especially under

stringent regulatory environments. However, this relationship is not

universal and is mediated by factors such as industry sector and

organizational culture (Post & Byron, 2015).

− The percentage of independent women shows a stronger correlation with

ROE than the overall percentage of women on boards of directors.

Recent literature distinguishes between symbolic and functional diversity. Independent

female directors, due to their lack of ties to executive management, are perceived as

more effective in monitoring and safeguarding stakeholders’ interests (Grosvold et al.,

2020). Studies such as Bernile et al. (2018) suggest that qualified diversity — which

combines gender and independence — has a more robust impact on financial

performance.

− Companies that demonstrate strong corporate governance practices —

particularly regarding gender diversity and board independence — are

generally associated with higher market valuation.

Market valuation is influenced not only by financial performance but also by perceptions

of governance quality. Recent studies have shown that institutional investors tend to

positively assess companies with diverse boards, associating them with lower

reputational risk and greater sustainability (García-Sánchez, Martínez-Ferrero, & García-

Meca, 2021; Liu, Wei, & Xie, 2022; Kassinis, Vafeas, & Chatzidakis, 2022). The market

valuation of firms has become increasingly associated with board diversity, particularly

the presence of independent female directors. According to Sipolatti (2023) “gender

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diversity on corporate boards is positively associated with the market’s perception of firm

value.” Compliance with best practices in board diversity is perceived as a signal of

organizational maturity and social responsibility (Zhang, Li, & Wang, 2023).

Recent European regulatory analyses reinforce the shift from symbolic to substantive

compliance, highlighting independence and gender balance as central governance

indicators. Reports from the European Commission (2024) and ESMA (2025) show that

board independence, particularly the presence of independent female directors, is

increasingly used as a proxy for governance quality and organisational maturity. This

evolving regulatory landscape strengthens the theoretical foundations of the Sandwich

Effect by positioning independent women at the intersection of compliance obligations

and market expectations.

Recent literature (2022–2025) highlights the distinction between symbolic and functional

diversity, with independent women playing a stronger monitoring, oversight and

governance‑enhancing role. Studies such as Agyemang et al. (2022), McGuire & Sheridan

(2023), Gouia et al. (2024) and Chang & Kang (2024) provide evidence that independent

female directors exert disproportionate influence on audit committee effectiveness,

stakeholder engagement and ESG orientation. These findings support the conceptual

foundation of qualified diversity, reinforcing the theoretical underpinnings of the

Sandwich Effect.

The “Sandwich Effect” in Corporate Governance

Recent scholarship highlights that women directors operate under dual external and

internal pressures. Studies by Bear, Rahman, and Post (2010), Ahern & Dittmar (2012)

and Stoeckl & Luedicke (2024) show that female directors are simultaneously subject to

heightened market scrutiny and internal governance expectations. Independent female

directors act as signaling agents for transparency, ethical conduct and ESG alignment.

These dynamics reinforce the conceptual logic of the Sandwich Effect, which captures

this dual tension and its impact on governance behavior and stakeholder perception.

The discussion surrounding the presence of women on corporate boards has gained

increasing prominence in academic literature, supported by various theoretical

frameworks such as Agency Theory, Stakeholder Theory, Resource Dependence Theory,

and Tokenism Theory. Emerging approaches argue that gender diversity in governance

bodies can contribute to more effective oversight, reduce agency conflicts, and promote

more ethical and transparent corporate practices.

In the organizational context, gender diversity is increasingly viewed not only as a matter

of social justice but also as a strategic factor for value creation and innovation. The

literature highlights that more diverse boards foster a culture of collaboration, enhance

the plurality of experiences and perspectives, and contribute to more robust decision-

making processes that are better aligned with the expectations of various stakeholders.

Moreover, the implementation of gender quotas and board independence requirements

has been adopted as a mechanism to accelerate female representation and to respond

to market demands and the principles of good corporate governance.

The “Sandwich Effect” introduces a novel approach to analyzing the role of women on

the boards of publicly listed companies. In this model, female board members are

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positioned at the intersection of two opposing forces: on one side, external market

pressures that increasingly demand gender diversity and associate inclusion with

improved financial performance; on the other, internal regulatory requirements and

corporate governance best practices, such as gender quotas and board independence

criteria, which organizations are expected to comply with. This dual pressure places

women directors in a central and strategic position, as they must navigate and reconcile

these external expectations and internal obligations, thereby amplifying their influence

and symbolic significance within corporate governance structures.

This dual pressure creates a unique context in which women—particularly independent

directors—are simultaneously agents of change and subjects of scrutiny. This study

operationalizes the concept by analyzing variables such as Return on Equity (ROE),

market valuation, the percentage of women on the board, the proportion of independent

female directors, and the degree of compliance with gender quota regulations.

The study assesses whether the presence of women—particularly independent female

directors—has a positive impact on firms’ financial performance, and whether compliance

with corporate governance best practices is valued by the market. The “Sandwich Effect”

thus offers an integrated perspective on the dynamics of gender, regulation, and

performance, contributing to the ongoing debate on the effectiveness of diversity policies

and value creation within organizations. Figure 1 illustrates the conceptual model of the

“Sandwich Effect.”

Figure 1. “Sandwich Effect”

Source: Developed by the authors.

The “Sandwich Effect” emerges as an innovative approach to analyzing the role of women

on corporate boards, positioning them between external market pressures and internal

regulatory and governance requirements. This perspective enables an integrated

assessment of whether female representation—particularly independent women

directors—effectively contributes to organizational performance and enhances firms’

market valuation.

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Methodology

This research adopts a quantitative approach with a descriptive and correlational design,

aiming to analyze the relationship between gender diversity on corporate boards and the

presence of independent female directors with financial performance and market

valuation of companies listed on the PSI index as of December 31, 2024. The

methodology was developed in three main stages:

(i) Literature Review; The theoretical framework was built based on Empirical

studies, published between 2003 and 2024 were prioritized, with

emphasis on research addressing gender diversity, the presence of

independent women on boards, and the impact of these variables on

financial performance and market valuation. This review supported the

formulation of three hypotheses and framed the conceptual model of the

study, including the theoretical model of the “Sandwich Effect.”

(ii) Data Collection: Data were collected from official and public sources,

namely: Corporate Governance Reports, 2024 Annual Reports and

Accounts, and information provided by the CMVM (Portuguese Securities

Market Commission).Variables extracted include: percentage of women

on the board, percentage of independent women, compliance with gender

quotas (based on the European Directive 2022/2381 and the CMVM

Corporate Governance Code), Return on Equity (ROE), and stock price.

(iii) Statistical Analysis: The statistical analysis applying techniques

appropriate to the nature of the variables and the sample size. Data

processing was performed using SPSS (Statistical Package for the Social

Sciences).

The methodological procedure followed these steps:

Normality Tests (Shapiro-Wilk): Applied to verify the distribution of the variables under

study, supporting the choice of subsequent statistical tests.

Hypotheses H1 and H2: To test the association between gender diversity on boards and

the presence of independent female directors with firms’ financial performance

(measured by Return on Equity – ROE), Pearson correlation and simple linear regression

were used, as both variables showed distributions compatible with normality.

Hypothesis H3: To assess whether companies that comply with corporate governance

best practices—regarding gender diversity and independence of non-executive

members—are more highly valued by the market, the closing stock price (as of 31-12-

2024) was compared between companies that comply and do not comply with gender

quota requirements and the number of independent members. After verifying normality,

the Student’s t-test for independent samples was applied (or, if normality was not

confirmed, the non-parametric Mann-Whitney test).

The analysis was conducted without control variables, focusing exclusively on the

relationship between gender diversity and financial performance (ROE) or market

valuation (stock price), according to the hypothesis under examination.

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Sample, Hypotheses, Variables, and Conceptual Model

Sample: The study focuses on the 15 companies listed on the PSI (Euronext Lisbon Stock

Index) as of December 31, 2024. (see table 1)

Table 1. PSI Companies as of December 31, 2024

Company

Ticker Market

Galp

GALP

J.Martins

JMT

EDP

Altri

ALTR

BCP

Navigator

NVG

Mota Engil

EGL

REN

Semapa

SEM

Sonae

SON

NOS

EDP Renováveis

EDPR

C. Amorim

COR

Ibersol

IBS

CTT

Source: Developed by the authors

Hypotheses: H1: The presence of women on corporate boards is positively associated

with Return on Equity (ROE); H2: The percentage of independent female directors has a

stronger correlation with ROE than the overall percentage of women on the board: H3:

The presence of independent female directors on boards (binary variable CBPCG) is

positively associated with the percentage of women on boards (MUC) and with market

performance (COT).

Justification for H1: The growing attention to gender diversity on corporate boards

reflects the perception that the inclusion of women can bring tangible benefits to firms’

performance. In the context of the PSI, the presence of women on boards is considered

a factor that may contribute to greater diversity of perspectives, improved decision-

making, and increased sensitivity to market needs. International studies suggest that

more diverse boards tend to be more innovative and adopt more effective management

practices, which can translate into better financial results, measured here by ROE.

Therefore, Hypothesis H1 is based on the premise that gender diversity is not merely a

regulatory requirement but can serve as a driver of value creation for companies.

Justification for H2: The concept of the “Sandwich Effect” highlights the strategic role of

women as intermediaries between market pressures and institutional demands.

However, not all female board members perform the same function. Independent female

directors, by virtue of not being affiliated with executive management or major

shareholders, tend to exercise more rigorous and impartial oversight. This enhances

corporate governance practices and safeguards the interests of a broader range of

stakeholders. The literature consistently identifies board independence as a key

determinant of effective monitoring and control, which may amplify the positive impact

of gender diversity on financial performance. Therefore, it is expected that the proportion

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of independent female directors will exhibit a stronger relationship with Return on Equity

(ROE) than the mere presence of women on the board.

Justification for H3: Market valuation reflects not only financial performance but also

investors’ perceptions regarding the quality of corporate governance and the

sustainability of business practices. Compliance with good Corporate Governance

principles—particularly in terms of gender diversity and board independence—is

increasingly viewed as a signal of commitment to transparency, fairness, and social

responsibility. These attributes are progressively valued by investors, especially

institutional ones, and by the market at large, which associates diversity with a lower

likelihood of reputational risks and a greater capacity to adapt to regulatory and societal

demands. Accordingly, Hypothesis H3 is grounded in the notion that adherence to best

practices in Corporate Governance may positively influence the market valuation of listed

companies. Table 2 presents the variables selected for the validation of the research

hypothesis

Table 2. Identification of Variables for Each Hypothesis

Hypothesis

Dependent Variable

Independent Variable(s)

Return on Equity (ROE)

% of women on the board

(MUC)

Return on Equity (ROE)

% of independent women

(MUI),

% of women on the board

(MUC)

% of women on the board

(MUC)

Company Valuation (COT)

Compliance with Corporate

Governance Best Practices

(CBPCG),

Source: Developed by the authors

Description of Variables

(ROE) Return on Equity: is a financial indicator that measures a company’s ability to

generate profit from the resources invested by its shareholders. In Hypothesis H1, ROE

is used to test whether the presence of women on boards is associated with improved

financial performance. In Hypothesis H2, ROE enables a comparison between the impact

of independent female directors and the overall percentage of women, identifying which

of these dimensions is more relevant to profitability.

(COT) Stock Price; refers to the closing price of shares traded on the market as of

December 31, 2024. In Hypothesis H3, this variable reflects the company’s market

valuation. By analyzing whether compliance with Corporate Governance best practices

influences stock price, the study seeks to understand whether the market (i.e., investors)

positively values companies that adopt policies promoting board diversity and

independence.

(MUC) % of Women on the Board of Directors: refers to the proportion of women holding

seats on a company’s board, relative to the total number of board members, expressed

as a percentage. In Hypothesis H1, the literature suggests that more diverse boards may

make more balanced, innovative, and market-sensitive decisions, which can be reflected

in improved financial performance (ROE). In Hypothesis H2, by separately analyzing the

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total percentage of women and the percentage of independent female directors, it

becomes possible to assess whether mere female presence or their independence has

distinct impacts on company performance as measured by ROE. In Hypothesis H3, the

percentage of women on boards is evaluated alongside compliance with Corporate

Governance best practices to analyze its influence on market valuation.

(MUI) % of Independent Women: refers to the proportion of women serving as

independent directors on a company’s board, expressed as a percentage. Board member

independence is one of the pillars of good Corporate Governance practices and is highly

valued by regulators, investors, and academic literature. In Hypothesis H2, this variable

allows for a comparison of whether the percentage of independent female directors has

a stronger correlation with ROE than the overall percentage of women, deepening the

analysis of the true impact of qualified diversity.

(CBPCG) Compliance with Corporate Governance Best Practices: This variable represents

the adoption of good governance practices regarding gender quotas and the presence of

independent female directors on company boards. It is operationalized as a binary

variable (CBPCG = 1 when the board includes at least one independent female director,

in accordance with Corporate Governance best practices; CBPCG = 0 when there are no

independent female directors on the board).

Conceptual Model

Figure 2. Conceptual Model of the Study

Source: Developed by the authors

Figure 2 presents the conceptual model of the current study, illustrating the proposed

relationships between the independent and dependent variables within the scope of the

formulated hypotheses.

In Hypothesis H1, the impact of the percentage of women on the board of directors (MUC)

on financial performance, measured by Return on Equity (ROE), is analyzed. In

Hypothesis H2, the combined influence of the percentage of independent women (MUI)

and the total percentage of women on the board (MUC) on ROE is assessed, allowing a

comparison between the effect of gender diversity and the independence of female

directors. Finally, Hypothesis H3 examines the relationship between compliance with

Corporate Governance best practices (CBPCG) and the market valuation of companies,

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represented by stock price (COT) and the integration of women into board composition.

The arrows in the figure highlight the direction of the analyzed impacts, providing a clear

and systematic view of the research hypothesis structure.

Results

Figure 3 shows that, from the analysis of companies listed on the PSI, 10 out of the 15

companies examined have 33.33% or more women on their boards of directors. This

threshold, often associated with minimum parity targets, allows the identification of a

subset of companies that, at first glance, demonstrate a stronger commitment to gender

diversity. Three companies — Navigator, Semapa, and CTT — stand out for having 100%

independent women, suggesting a more robust integration of female profiles with

autonomy from management. In contrast, Ibersol, despite having 40% women on its

board, does not include any independent female directors, raising questions about the

actual independence and functional diversity of its board.

Figure 3. Study Data: MUC and MUI

Source: Developed by the authors

Women’s Participation on Boards of Directors of PSI-Listed Companies

The European Directive 2022/2381, commonly referred to as “Women on Boards,” sets

clear targets to increase female representation on the boards of publicly listed companies

within the European Union. By 30 June 2026, the Directive requires that 40% of non-

executive director positions be held by members of the underrepresented sex (typically

PSI Listed

Companies

Galp 36,84% 66,67%

J.Martins 36,36% 66,67%

EDP 38,10% 66,67%

Altri 31,25% 75,00%

BCP 25,00% 33,33%

Navigator 33,33% 100,00%

Mota Engil 33,33% 60,00%

REN 40,00% 85,71%

Semapa 33,33% 100,00%

Sonae 23,08% 33,33%

NOS 15,38% 25,00%

EDP Renováveis 33,33% 60,00%

C. Amorim 27,27% 25,00%

Ibersol 40,00% 0,00%

CTT 35,71% 100,00%

MUC

MUI

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women), and at least 33% of all director positions (executive and non-executive

combined) meet this criterion (European Union, 2022).

Figure 4. Women’s Share on Boards of PSI-Listed Firms (Dec. 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

Figure 4 shows that most companies listed on the PSI as of December 2024 already

comply with the gender quota of 33.33% established by European Directive 2022/2381,

whose full implementation is scheduled for June 2026. This Directive aims to ensure a

minimum representation of 33% of members of the underrepresented sex on the boards

of publicly listed companies.

The red line represents the minimum quota of 33.33% female representation, as

established by the European directive on gender balance in the boards of publicly listed

companies. It provides a clear visualization of the degree of alignment of Portuguese

listed firms with European regulatory requirements, highlighting both cases of greater

balance and situations where progress is still needed.

Regarding the number of independent directors required on the boards of publicly listed

companies, the Portuguese Corporate Governance Code issued by CMVM, revised in 2023

and effective in 2024, maintains one of its most relevant recommendations for sound

governance: at least one-third (33.33%) of non-executive directors should be

independent, and this number must always be plural. Although the Code is voluntary, it

follows the “comply or explain” principle, meaning companies must either comply or

publicly justify non-compliance. Figure 5 below illustrates the degree of compliance with

this recommendation among PSI-listed companies as of December 2024

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Figure 5. Degree of Compliance with Board Independence in PSI Companies (December 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

It should be noted that 6 companies do not meet this recommendation (Galp, EDP, Altri,

Semapa, NOS, and Ibersol), although their percentages are quite close to the 33.33%

threshold indicated in the chart by the dashed line.

Figure 6. Percentage of Independent Women within Total Independents PSI (December 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

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Regarding independent women among the total number of independents, Figure 6 shows

that the integration of independent women, relative to the total of independents, is quite

high, with some companies (Navigator, Semapa, and CTT) having 100% of independent

members as women. The dashed line visually highlights the cases of full female

representation among independents.

The ROE (Return on Equity) variable is one of the most relevant financial indicators for

assessing a company’s financial performance, reflecting its ability to generate returns for

shareholders based on the equity invested. In the context of the analyzed sample,

composed of companies listed on the PSI index, the statistical analysis of ROE reveals an

average of 0.106 and a median of 0.104, indicating a relatively symmetrical distribution

around low profitability value. The standard deviation of 0.063 shows a moderate

dispersion of values, suggesting some heterogeneity in company performance. The

minimum observed value was -0.046, indicating the existence of at least one company

with negative profitability, while the maximum value reached 0.221, representing the

best performance in the sample. The ROE distribution, represented graphically through

a histogram with a density curve, shows a significant concentration of companies with

values between 0.05 and 0.15, while the boxplot highlights the presence of outliers,

particularly at the lower end of the distribution (see Figure 7). These results are

particularly relevant for analyzing the effectiveness of Corporate Governance, especially

regarding the presence of independent women on boards of directors, as they allow for

exploring potential correlations between diversity and financial performance.

Figure 7. ROE: Histogram and Boxplot - PSI (December 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

The CBPCG variable was constructed as a binary indicator to assess the degree of

compliance of companies with minimum corporate governance best practice criteria. It

posits that the presence of independent women on boards of directors would be positively

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associated with the percentage of women on the Board of Directors (MUC) and stock

market performance (COT), reflecting greater sensitivity to market pressures.

Results for H1: Association between the Percentage of Women on the Board of Directors

and ROE - To test the hypothesis that the percentage of women on the Board of Directors

is positively associated with the financial performance of PSI companies, measured by

Return on Equity (ROE), an exploratory statistical analysis was conducted without control

variables, based on official 2024 data.

The average percentage of women on the Board of Directors was 32.2% (standard

deviation: 6.8%), with values ranging from 15.4% to 40.0%. In turn, the average ROE

was 10.6% (standard deviation: 6.3%), varying between -4.6% and 22.1%. The

Shapiro-Wilk normality tests indicated that both variables exhibit a distribution

compatible with normality (p > 0.05), allowing the application of Pearson’s correlation to

assess the association between them. The correlation coefficient obtained was -0.196 (p

= 0.485), suggesting a weak, negative, and statistically non-significant relationship

between the percentage of women on the Board of Directors and ROE.

The simple linear regression reinforced these findings, presenting a coefficient for the

percentage of women on the Board of Directors of -0.1788 (p = 0.485), without statistical

significance. The model explained only 3.8% of the variation in ROE (R² = 0.038),

indicating a very limited explanatory capacity. Figure 8 illustrates the relationship

between the variables and the fitted regression line.

Figure 8. Linear Regression between MUC and ROE – PSI (December 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

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The results do not reveal a statistically significant association between the proportion of

women on the board of directors and the Return on Equity (ROE) of PSI-listed companies

in 2024. The statistical analysis conducted did not support the validation of Hypothesis

1. Based on the available data for 2024, no empirical evidence was found to substantiate

the assumption that a higher representation of women on boards is linked to improved

financial performance, as measured by ROE, among PSI firms.

Results for H1: Association between the Percentage of Independent Women on the Board

of Directors and ROE - To test the hypothesis that the percentage of independent women

on the board of directors has a positive impact on the financial performance of PSI-listed

companies, measured by Return on Equity (ROE), a statistical analysis was conducted

without control variables, based on official 2024 data. The average percentage of

independent women was 59.8% (standard deviation: 30.8%), ranging from 0% to 100%.

The average ROE of the companies was 10.6% (standard deviation: 6.3%), with values

between -4.6% and 22.1%. Normality tests (Shapiro–Wilk) indicated that both variables

follow a distribution compatible with normality (p > 0.05). Therefore, Pearson’s

correlation was applied to assess the association between the variables. The correlation

coefficient obtained was 0.285 (p = 0.303), indicating a positive but weak and statistically

non-significant relationship between the percentage of independent women on the board

of directors and ROE.

The simple linear regression reinforced these findings, presenting a coefficient for the

percentage of independent women of 0.0579 (p = 0.303), without statistical significance.

The model explained only 8.1% of the variation in ROE (R² = 0.081). The following figure

illustrates the relationship between the variables and the fitted regression line.

Figure 9. Simple Linear Regression. MUI e ROE - PSI (December 2024)

Source: Data processed by the authors in SPSS, based on publicly available company information

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Overall, the empirical evidence does not indicate a statistically significant relationship

between the proportion of independent female directors on corporate boards and the

return on equity (ROE) of PSI-listed firms in 2024. The statistical analysis did not allow

for the validation of Hypothesis 2, which posited a positive impact of the proportion of

independent female directors on the board of directors on the ROE of PSI-listed

companies. Based on the data available for 2024, no empirical evidence was found to

support the hypothesis that a higher presence of independent women on the board is

associated with improved financial performance, as measured by ROE, among PSI firms

Results for H3: The good governance practice regarding the presence of independent

women on boards of directors (binary variable CBPCG) is positively associated with the

proportion of women on boards (MUC) and with market performance (COT) - The

statistical analysis revealed a strong correlation between CBPCG and MUC (r = 0.83),

suggesting that boards including independent women tend to exhibit a higher proportion

of women in key corporate roles. Furthermore, a moderate correlation was observed

between CBPCG and COT (r = 0.40), indicating a positive association between the

presence of independent women on boards and firms’ market performance.

These results were complemented by mean comparison tests (Student’s T-test), which

revealed statistically significant differences between groups with and without the

presence of independent women on boards, thereby reinforcing the validity of the

observed association. Figure 10 illustrates the distribution of the percentage of MUC

across CBPCG groups, revealing a clear concentration of higher values in cases where

the good governance practice is present.

Figure 10. Distribution of MUC by the Presence of Independent Women on Boards (CBPCG)

Source: Data processed by the authors in SPSS, based on publicly available company information

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Figure 11 shows the distribution of market performance (COT), where a tendency toward

higher values can also be observed in cases with CBPCG = 1, although with greater

dispersion.

Figure 11. Market Performance (COT) by Presence of Independent Women on the Board

(CBPCG)

Source: Data processed by the authors in SPSS, based on publicly available company information

Discussion

The empirical evidence suggests that the incorporation of independent female directors

may serve a dual purpose: fostering meaningful gender diversity at the strategic level

and enhancing organizational adaptability to market expectations. In this regard,

Hypothesis H3 appears to be supported, as the findings indicate a positive association

between the presence of independent women on boards and both female representation

and market valuation. These results align with the theoretical assumptions embedded in

the “Sandwich Effect” construct, which positions independent female directors as

mediators between regulatory obligations and market dynamics.

Conversely, Hypotheses H1 and H2, which examined the relationship between gender

diversity – both overall and in terms of independence – and financial performance (ROE),

did not yield statistically significant results. This outcome suggests that gender diversity

alone may not guarantee superior financial performance within PSI-listed firms, a finding

consistent with prior literature emphasizing the contextual nature of this relationship.

Rather than dismissing the relevance of diversity, these results underscore the

complexity of its impact, which may be mediated by factors such as industry

characteristics, governance culture, and broader economic conditions.

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Taken together, the findings point to a nuanced distinction between the effects of gender

diversity on financial performance and on market valuation. While the data do not support

a direct link between diversity and profitability, they indicate that governance practices

promoting inclusivity – particularly through the integration of independent female

directors – are positively perceived by the market. This reinforces the theoretical

proposition that qualified diversity, aligned with sound governance principles, can

enhance reputational strength and investor confidence.

Overall, the results provide empirical support for the “Sandwich Effect” as a conceptual

lens for understanding the strategic role of women on boards. By framing female directors

as intermediaries between institutional requirements and market pressures, the model

offers a valuable perspective for future research and for organizations seeking to balance

compliance with competitive advantage.

Conclusions

This study focused on the role of women on the boards of PSI-listed companies, with

particular emphasis on independent directors, in light of good corporate governance

practices and European regulatory requirements. The research was guided by four

objectives, directly linked to the hypotheses formulated:

The first objective was linked to Hypothesis H1, which was not statistically validated. This

outcome is consistent with prior academic literature (Adams & Ferreira, 2009; Joecks et

al., 2013), which acknowledges that the impact of gender diversity on financial

performance is not universal, but rather mediated by factors such as institutional context,

organizational culture, and industry characteristics. The second objective was linked to

Hypothesis H2, which was also not statistically validated. Nevertheless, the analysis

enabled a deeper distinction between symbolic diversity and functional diversity,

reinforcing the theoretical relevance of independence as an effective monitoring factor,

in line with the findings of Grosvold et al. (2020) and Bernile et al. (2018).

The confirmation of Hypothesis H3, linked to the third research objective, reinforces the

theoretical proposition that market valuation is sensitive to governance practices

promoting inclusivity, sustainability, and reputational strength, as documented in recent

empirical studies (García-Sánchez et al., 2021; Liu et al., 2022; Zhang et al., 2023)

The fourth objective – introducing and framing the ‘Sandwich Effect’ concept – was

successfully achieved. This construct positions female board members as mediators

between regulatory requirements and market pressures, offering an integrated and

innovative perspective on their strategic role within boards. Grounded in theoretical

approaches, the model enriches the academic debate on functional diversity and

corporate governance

For academia, this study proposes a replicable conceptual model that can be tested

across different institutional and cultural contexts, thereby contributing to the theoretical

advancement of research on board diversity and effectiveness. For companies, the

findings underscore the importance of integrating independent female directors as part

of a broader strategy aimed at enhancing corporate reputation, ensuring transparency,

and aligning with regulatory requirements. For the economy, the study highlights that

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inclusive governance practices are recognized by the market and may positively influence

firm valuation and investor confidence.

This research suggests that gender diversity alone does not guarantee improved financial

performance. However, qualified diversity aligned with sound governance practices –

particularly through the presence of independent female directors – is valued by the

market and contributes to more ethical, effective, and sustainable governance. The

concept of the “Sandwich Effect” emerges as a relevant theoretical tool for understanding

the dynamics of power, regulation, and reputation within corporate boards, paving the

way for future research into the strategic role of women in creating organizational value.

Limitations and Directions for Future Research

Despite the contributions of this study to the understanding of the impact of gender

diversity on the boards of PSI-listed companies, certain limitations must be

acknowledged, as they may constrain the generalizability of the findings. First, the

analysis focused exclusively on the 15 companies comprising the PSI index, which

restricts the sample scope and may not reflect the broader reality of other Portuguese

firms or different market contexts. Second, the study adopted a limited temporal horizon,

focusing solely on data from 2024, which prevents the identification of trends or long-

term effects associated with gender diversity.

Additionally, the absence of control variables – such as firm size, industry sector,

ownership structure, or broader macroeconomic conditions – may limit the robustness of

the conclusions, as these factors can also influence both board composition and corporate

financial performance or market valuation. Furthermore, the study’s exclusively

quantitative approach, based on indicators such as the percentage of women on boards,

return on equity (ROE), and year-end stock prices, did not allow for the exploration of

relevant qualitative dimensions. These include the actual role of women in decision-

making processes, organizational culture, and internal board dynamics, which are crucial

for a more comprehensive understanding of how gender diversity translates into

governance effectiveness.

Future research is encouraged to expand the sample to include companies from other

indices or markets, as well as non-listed firms, in order to obtain a more comprehensive

view of the impact of gender diversity. Adopting a longitudinal approach would allow for

the monitoring of diversity and performance over time, capturing cumulative or

contextual effects. Moreover, incorporating control variables and employing mixed-

methods approaches – combining statistical analysis with interviews, case studies, or

document analysis – could enrich the understanding of women’s roles on boards,

particularly within the framework of the “Sandwich Effect” model. Comparative studies

between Portugal and other European countries could also shed light on national

specificities and the influence of different regulatory approaches in promoting gender

diversity. Such studies would further enable the testing of the applicability and relevance

of the “Sandwich Impact” concept across diverse institutional contexts, contributing to

its theoretical development and empirical validation.

The “Sandwich Effect” concept proves particularly relevant as a theoretical tool for

understanding the structural tensions that shape the role of women on corporate boards.

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By positioning these women between external market pressures and internal demands

for good governance practices, the concept enables a critical and multifaceted reading of

their actions and influence. The “Sandwich Effect” holds significant potential for

operationalization in future research through the inclusion of additional variables, such

as board turnover, functional diversity, or ESG sustainability indicators. This theoretical

flexibility reinforces the concept’s utility as an analytical lens for exploring the complex

dynamics of power, representation, and performance within corporate governance

structures.

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