274 Chapter 33
Genetic influence
Most psychological traits have been found to have a heritability of around
40–60%, that is, genetic differences between individuals account for
roughly half of the observed variance in a given population. Disruptive
behavioural problems are one likely exception to this rule, with most
(but not all) studies showing a smaller genetic contribution to this sort
of behaviour. At the opposite extreme, liability to autism may have a
heritability of over 90%.
Effects of shared environment
In the 1980s, some behavioural geneticists made the dramatic claim,
based mainly on twin studies, that shared family environment has little
effect on many personality traits. A review in 1987 stated the matter
particularly forcibly: ‘What parents do that is experienced similarly by
their children does not have an impact on their behavioural development.’
The gist of the argument is that twin and adoption findings show that
family resemblances are almost all attributable to shared genes rather than
shared environment; adoptees hardly resemble their adoptive relatives
at all in personality traits. Even at the height of these claims, disruptive
behavioural problems stood out as important exceptions to the general
rule, with most studies suggesting that shared environment is a major
reason for disruptive behaviour running in families. However, there are
problems with these generalisations, as discussed below.
Effects of non-shared environment
If genes typically explain about half of the variance for most psycho-
logical traits, and if shared environmental effects are often weak or
absent, what explains the rest of the variance? The popular answer is
‘non-shared environment’, suggesting that children and adolescents are
particularly influenced by the experiences that they do not share with
their siblings. It is certainly plausible, for example, that when parents pay
more attention to one sibling than another, this is more wounding than
when both siblings get less parental attention than the ‘average child’.
Mental health professionals have long been interested in the effects of
scapegoating and favouritism, which are specific instances of non-shared
environmental effects. Focusing on each individual’s unique experience
of the family environment can be very helpful clinically. At the same
time, it is unclear how powerful these non-shared environmental effects
really are. It is wrong to suppose that if genes and shared environ-
ment only account for half of the variance on any given psychological
trait, then the impact of non-shared experiencesmust account for the
other half. As described below, non-genetic variance can have other
explanations.