It is often discussed how strange it is that people are so different to their family members. After all, shared genes and a shared home environment should lead to strong family resemblance, right?
Behavioural genetic research has found that once genetic similarity is accounted for, individuals within a family are on the whole no more alike than if they were selected at random. But why is this the case?
Phenotypic variance for any complex trait is never fully explained by genetic influences, thus any remaining variance is attributed to environmental influences. While shared aspects of an environment will also contribute to resemblance between family members, it is those unique environmental experiences, or the nonshared environment, that can help to explain differences within a family.
“it is those unique environmental experiences, or the nonshared environment, that can help to explain differences within a family”
After accounting for variance due to heredity, shared environment, and error of measurement, unique environmental variance can be quantified. Adoption studies have been particularly useful in helping us to understand the importance of unique environmental effects. For example, if the correlation between genetically unrelated individuals adopted together is zero, this indicates that environmental variation for that particular trait would be nonshared.
The twin design is also useful for estimating the importance of unique environmental experiences. Given that identical twins share 100% of their genetic material, any differences within pairs of identical twins, reared in the same environment, can be attributed to the nonshared environment. More complex processes such as epistasis and gene-environment correlations and interactions are accounted for because these identical twins share all sources of genetic variance. This is important to note given that, for siblings who are not identical twins, differential effects of experience on outcomes could be due to gene-environment correlations (Jaffee & Price, 2007).
Specific sources of nonshared environmental influences thought to affect development include the composition of a family, birth order, sibling interaction or extrafamilial influences such as accidents and the peer group (Plomin, DeFries, Knopik & Neiderhiser, 2013). Less obvious sources of nonshared environmental effects include the prenatal environment, cytoplasmic changes, and DNA changes that are not transmitted intergenerationally (Plomin & Daniels, 2011).
“research finds that parents treat their children differently, and divorce is often experienced differently by children in the same family, thus these experiences can be viewed as unique”
Indeed, any environmental factor can be viewed in terms of its contribution to unique environmental effects, as long as as it can be assessed separately for each individual. For example, parenting or divorce are ostensibly part of the shared environment; however, siblings may react differently to these events. In fact, research finds that parents treat their children differently, and divorce is often experienced differently by children in the same family, thus these experiences can be viewed as unique (Reiss, Neiderhiser, Hetherington & Plomin, 2000; Hetherington & Clingempeel, 1992).
Overwhelmingly, it appears that unique environmental factors represent the most important source of environmental variation relevant to psychological phenomena. While genetic influence on individual differences for most complex traits is substantial, it is rare to find evidence that more than half of the variance for complex traits is due to genetic differences among individuals. Across a range of studies related to psychological traits such as personality, psychopathology, including schizophrenia and depression, and physical illnesses, such as type two diabetes and stomach ulcers, half of the variance in these traits were accounted for by unique environmental factors (Loehlin & Nichols, 1976; Turkheimer & Waldron, 2000; Dunn & Plomin, 1990).
“it is rare to find evidence that more than half of the variance for complex traits is due to genetic differences among individuals”
The finding that unique environmental factors predominantly explains differences between children growing up in the same family remains unchallenged. The overarching message is not that family experiences do not matter, rather that researchers must consider that relevant experiences are specific to each child rather than general to all children within a family.
- Dunn, J., & Plomin, R. (1990). Separate lives: Why siblings are so different. New York: Basic Books.
- Hetherington, E., & Clingempeel, W. (1992). Coping with marital transitions: A family systems perspective. Monographs of the Society for Research in Child Development, 57, 1-242.
- Jaffee, S. R., & Price, T. S. (2007). Gene–environment correlations: A review of the evidence and implications for prevention of mental illness. Molecular Psychiatry, 12(5), 432.
- Loehlin, J., & Nichols, R. C. (1976). Heredity, environment and personality: A study of 850 sets of twins. Austin, TX: University of Texas Press.
- Plomin, R., & Daniels, D. (2011). Why are children in the same family so different from one another?. International Journal of Epidemiology, 40(3), 563-582.
- Plomin, R., DeFries, J. C., Knopik, V. S., & Neiderhiser, J. M. (2013). Behavioral genetics (6th ed.). New York: Worth Publishers.
- Reiss, D., Neiderhiser, J., Hetherington, E. M., & Plomin, R. (2000). The relationship code: Deciphering genetic and social patterns in adolescent development. Cambridge, MA: Harvard University Press.
- Turkheimer, E., & Waldron, M. (2000). Nonshared environment: a theoretical, methodological, and quantitative review. Psychological Bulletin, 126, 78-108.