Research Article: Do MZ twins have discordant experiences of friendship? A qualitative hypothesis-generating MZ twin differences study

Date Published: July 20, 2017

Publisher: Public Library of Science

Author(s): Kathryn Asbury, Nicola Moran, Robert Plomin, Igor Branchi.


Using a qualitative monozygotic (MZ) twin differences design we explored whether adolescent MZ twins report discordant peer relationships and, if so, whether they perceive them as causes, consequences or correlates of discordant behaviour. We gathered free-response questionnaire data from 497 families and conducted in-depth telephone interviews with 97 of them. Within this dataset n = 112 families (23% of the sample) described discordant peer relationships. Six categories of discordance were identified (peer victimisation, peer rejection, fewer friends, different friends, different attitudes to friendship and dependence on co-twin). Participants described peer relationship discordance arising as a result of chance occurrences, enhanced vulnerability in one twin or discordant behaviour. Consequences of discordant peer relationships were seen as discordance in self-confidence, future plans, social isolation, mental health and interests. In all cases the twin with worse peer experiences was seen as having a worse outcome. Specific hypotheses are presented.

Partial Text

Behavioural genetic studies have confirmed that there are both genetic and environmental influences on human behaviour [1]. In the majority of cases the most influential environments are individual-specific, or non-shared, making us differ from those we are raised with [2–4]. However, non-shared environment (NSE), while recognised as a major source of behavioural variation, remains poorly understood and under-explored. This manuscript reports one strand of an unprecedentedly large qualitative monozygotic (MZ) twin differences study which was designed to address this dearth of understanding by taking an inductive approach to generating new, testable hypotheses about NSE [5]. We present findings related to peer relationships as one potential aspect of NSE.

This study was approved by the Institute of Psychiatry Ethics Committee (PNM/11/12-142).

Six categories of peer-relationship discordance were identified in questionnaire and/or interview data gathered from 112 families (See Table 1).

A substantial minority (23%) of participants in this wide-ranging study spontaneously described and discussed discordance in friendships and peer relationships when asked about within MZ twin pair differences. Their responses suggested six categories of discordance of which four (peer victimisation, peer rejection, fewer friends and different friends) can be interpreted as environmental variables. The other two categories were different attitudes to friendship and dependence on a co-twin, and these are more easily interpreted as behavioural variables, albeit with non-shared roots and flowers. Together they suggest avenues for future research into experiences of friendship as components of the non-shared environment.




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