Genetics and Human Behaviour: The Ethical Context
Environmental influences on variation
4.9 Environmental variance includes a proportion of variance that can be explained by shared or common environmental influences and a remaining proportion accounted for by nonshared environmental factors, random effects and error. The terms ‘shared’ and ‘nonshared’ environment refer to the effects of the environmental influence, not their origins (as previously assumed by many researchers); that is whether they increase or decrease similarity between family members for a given characteristic. For example, social disadvantage is an environmental factor to which all family members are exposed. However, its effects on a particular behaviour may appear as ‘non-shared’ in genetic analyses if social disadvantage has a different effect on each individual within the family; that is, if its effect is to enhance differences between family members. The categories of shared and non-shared environmental influences are statistical notions and each category contains many unidentified factors that are not necessarily specifically investigated in a quantitative research project.
4.10 Many studies in this field have reported that the proportion of variation in behavioural traits that can be attributed to factors shared by family members is relatively low. These findings have been used to claim that an individual’s shared environment, particularly the family, has little effect on his or her behaviour.(6) Interestingly, an exception to this finding is in the case of antisocial behaviour. Significant effects of shared environments are routinely reported in research in behavioural genetics in this area. Moreover, many researchers have now rejected the conclusion that the role of the family in affecting behaviour is unimportant.(7)