Towards a Greater Understanding of the Role of the Environment: A Systematic Review of Qualitative MZ Twin Differences Studies

Filip Marzecki; B. Kennath Widanaralalage; Nandini Nhandoh; Tom A.  McAdams; Yasmin Ahmadzadeh; Helena Zavos

doi:10.1007/s10519-025-10217-1

Towards a Greater Understanding of the Role of the Environment: A Systematic Review of Qualitative MZ Twin Differences Studies

Filip Marzecki, B. Kennath Widanaralalage, Nandini Nhandoh, Tom A. McAdams, Yasmin Ahmadzadeh, Helena Zavos

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Abstract

Background: The environment is an important influence in the development of human behaviours and health outcomes. However, one of the most consistent findings from behavioural genetic studies is that most environmental influences are not shared between members of the same family. A compelling way of investigating these non-shared environmental influences is by using a monozygotic (MZ) twin differences design. Quantitative MZ differences studies have uncovered systematic non-shared environmental factors, i.e., those acting according to a general pattern in a population, for many traits, but may be omitting idiosyncratic or distinctive factors and mechanisms. Qualitative MZ differences design provides an alternative. In this study design, identical twins discordant on an outcome are interviewed in depth about the origins and context of their discordance, providing an insight into distinctive lived experiences. Methods: We conducted a systematic review examining the results and methodological features of studies using qualitative data collection and analyses to investigate differences in identical twins' experiences and outcomes. We applied a narrative synthesis. Results: We identified seven studies, covering a range of phenotypes (e.g., anxiety or smoking) and participants (from children to older adults), which found a wide range of themes related to twins’ discordance. A major theme arising from the narrative synthesis was the role of personality and individual traits, e.g., confidence or sexual orientation, in explaining MZ twins’ discordant experiences and outcomes. Conclusions: Non-shared environmental factors are at least partly idiosyncratic and are therefore suitable for exploration with a qualitative research design, ideally in parallel with quantitative twin research in mixed-method research projects or programmes.

Original language	English
Journal	Behavior Genetics
DOIs	https://doi.org/10.1007/s10519-025-10217-1
Publication status	Published - 23 Feb 2025

Keywords

MZ twins
Qualitative
environment

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

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Marzecki-et-al-manuscript-v3Accepted author manuscript, 138 KBLicence: CC BY

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title = "Towards a Greater Understanding of the Role of the Environment: A Systematic Review of Qualitative MZ Twin Differences Studies",

abstract = "Background: The environment is an important influence in the development of human behaviours and health outcomes. However, one of the most consistent findings from behavioural genetic studies is that most environmental influences are not shared between members of the same family. A compelling way of investigating these non-shared environmental influences is by using a monozygotic (MZ) twin differences design. Quantitative MZ differences studies have uncovered systematic non-shared environmental factors, i.e., those acting according to a general pattern in a population, for many traits, but may be omitting idiosyncratic or distinctive factors and mechanisms. Qualitative MZ differences design provides an alternative. In this study design, identical twins discordant on an outcome are interviewed in depth about the origins and context of their discordance, providing an insight into distinctive lived experiences. Methods: We conducted a systematic review examining the results and methodological features of studies using qualitative data collection and analyses to investigate differences in identical twins' experiences and outcomes. We applied a narrative synthesis. Results: We identified seven studies, covering a range of phenotypes (e.g., anxiety or smoking) and participants (from children to older adults), which found a wide range of themes related to twins{\textquoteright} discordance. A major theme arising from the narrative synthesis was the role of personality and individual traits, e.g., confidence or sexual orientation, in explaining MZ twins{\textquoteright} discordant experiences and outcomes. Conclusions: Non-shared environmental factors are at least partly idiosyncratic and are therefore suitable for exploration with a qualitative research design, ideally in parallel with quantitative twin research in mixed-method research projects or programmes. ",

keywords = "MZ twins, Qualitative, environment",

author = "Filip Marzecki and Widanaralalage, {B. Kennath} and Nandini Nhandoh and McAdams, {Tom A.} and Yasmin Ahmadzadeh and Helena Zavos",

note = "Journal's Open Access note: This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.",

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doi = "10.1007/s10519-025-10217-1",

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T1 - Towards a Greater Understanding of the Role of the Environment: A Systematic Review of Qualitative MZ Twin Differences Studies

AU - Marzecki, Filip

AU - Widanaralalage, B. Kennath

AU - Nhandoh, Nandini

AU - McAdams, Tom A.

AU - Ahmadzadeh, Yasmin

AU - Zavos, Helena

N1 - Journal's Open Access note: This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

PY - 2025/2/23

Y1 - 2025/2/23

N2 - Background: The environment is an important influence in the development of human behaviours and health outcomes. However, one of the most consistent findings from behavioural genetic studies is that most environmental influences are not shared between members of the same family. A compelling way of investigating these non-shared environmental influences is by using a monozygotic (MZ) twin differences design. Quantitative MZ differences studies have uncovered systematic non-shared environmental factors, i.e., those acting according to a general pattern in a population, for many traits, but may be omitting idiosyncratic or distinctive factors and mechanisms. Qualitative MZ differences design provides an alternative. In this study design, identical twins discordant on an outcome are interviewed in depth about the origins and context of their discordance, providing an insight into distinctive lived experiences. Methods: We conducted a systematic review examining the results and methodological features of studies using qualitative data collection and analyses to investigate differences in identical twins' experiences and outcomes. We applied a narrative synthesis. Results: We identified seven studies, covering a range of phenotypes (e.g., anxiety or smoking) and participants (from children to older adults), which found a wide range of themes related to twins’ discordance. A major theme arising from the narrative synthesis was the role of personality and individual traits, e.g., confidence or sexual orientation, in explaining MZ twins’ discordant experiences and outcomes. Conclusions: Non-shared environmental factors are at least partly idiosyncratic and are therefore suitable for exploration with a qualitative research design, ideally in parallel with quantitative twin research in mixed-method research projects or programmes.

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DO - 10.1007/s10519-025-10217-1

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SN - 0001-8244

JO - Behavior Genetics

JF - Behavior Genetics

ER -

Towards a Greater Understanding of the Role of the Environment: A Systematic Review of Qualitative MZ Twin Differences Studies

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Keywords

UN SDGs

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