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

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. 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. 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. 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.