Nature and Nurture 277not necessarily mean that there is nothing in the shared environment
bringing out schizophrenia in those genetically predisposed to it. For
example, migration or discrimination may make one family member
schizophrenic, while another may get depressed, or cope.
5 It is now well documented that some environments are genetically
influenced, and this can lead to an overestimation of environmental
effects. For example, cross-sectional studies show that in households
where there are more books and where parents read more to their
offspring, the children have considerably higher reading ages. It might
be concluded that buying extra books and reading them to children will
lead to big improvements in the children’s reading ability, but it turns
out that a third factor, parental IQ, mediates a good deal of the effect.
While parents of higher IQ tend to read more to their children, their
children tend to have higher IQ and are better readers anyway. This is
not to say that increasing reading with children will not improve their
ability, but that the effects may be more modest that anticipated.
Genes and environment interact
The notion that either genes or environments can act alone is over-
simplistic: the two are always interacting. Through evolution, our genetic
make-up is designed to be sensitive to the likely environment. We need
to consider the mechanisms through which environments affect a given
genotype, and what the mechanisms are through which genes influence
the reaction to a given environment.
In the 1960s, longitudinal studies established that children’s devel-
opment was influenced by an interaction between temperament and
upbringing, with temperamentally placid babies being less affected by
insensitive parenting than temperamentally irritable babies. Subsequent
adoption studies have shown how genetic sources of variation in tem-
perament influence parenting, and interact with it. Observational studies
show that the disciplinary practices of adoptive parents are more harsh
and critical when their children have criminal birth parents, probably
because the children are harder to cope with because they have inherited
a tendency to more disruptive behaviour.
An influential Scandinavian adoption study examined which adoptees
were convicted for criminal behaviour as adolescents. Each adoptee was
classified according to both biological and psychosocial risk:
1 An adoptee was said to be at highcongenitalrisk if thebirth parentshad
criminal or alcoholic histories. Otherwise, congenital risk was said to
be low. Any risk transmitted from birth parents was probably genetic,
though the prenatal environment could also be relevant, for example,
if high alcohol consumption by the biological mother during pregnancy
influenced fetal brain development.
2 An adoptee was predicted to have receivedworse rearingif theadoptive
parentshad criminal or alcoholic histories. Otherwise, the adoptee was