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pre-existing differences between the participants who were and were
not exposed to the changed environment. While it is obviously not
ethical to expose children at random to adverse environments, it may
be ethical to randomise children to environmental improvements (for
example, if there are insufficient funds to offer this to everyone). Such
trials have shown, for example, that improving parenting leads to a large
reduction in antisocial behaviour, and that early cognitive stimulation
in deprived preschoolers leads to better adjustment and achievement in
adulthood.
3 Twin studies may underestimate environmental effects if genotypes
predispose individuals to environmental risks. For example, imagine a
gene that made individuals more likely to become addicted to cigarettes.
This would result in identical twins being more similar than non-
identical twins in their smoking habits. In fact, it would be a genetic
pathway to lung cancer – and yet it is the environmental risk (smoking)
that is the final cause of the cancer. Simply reporting that lung cancer
was highly heritability could easily result in the misleading view that
there was little or no point in trying to use environmental interventions
(such as taxing or banning cigarettes) to deal with what seems to be
a very heritable problem. Similar issues arise in child and adolescent
psychiatry. For instance, the combination of ADHD and a disruptive
behavioural disorder has a high heritability, but this does not mean
that environmental interventions are unimportant. That is because what
is heritable is primarily the ADHD, but the symptoms of ADHD often
evoke a style of hostile parenting that leads to the child or adolescent
developing disruptive behaviour too. It would therefore be wrong to
conclude that because heritability (as measured) is high, and shared
environment influences (as calculated) are low or non-existent, that
there are no treatments that might work by altering the environment.
In fact trials show that improving the parenting of children who have
both ADHD and disruptive behaviour can be very effective.
4 The distinction between shared and non-shared environments can lead
to misunderstandings – even among experts! As noted above, non-
shared environment is used to explain differences between family
members after genetic influences have been allowed for. Sometimes this
may indeed be due to non-shared influences. However, the difference
can also be due to the same, shared environmental influences affecting
individuals within the family differently, either directly or because they
perceive it differently. Thus, living with a hostile parent might, for
example, make one child anxious, another aggressive, leave a third
indifferent, and the fourth may be made stronger. In this case it is
the reaction to the environment that differs, not the environmental
influence per se. This, then, is an interaction between a shared en-
vironmental influence and the child’s susceptibility, but conventional
behaviour genetics would badge it as non-shared environment. To
take another example, the results of twin studies of schizophrenia
show that ‘non-shared’ environmental influences are considerable and
‘shared’ environmental influences are insignificant. Again, this does