Nature and Nurture 271Before reviewing a number of commonly encountered family adversities
(see Chapter 34), it is important to consider some general principles that
help tease out causal relationships and disentangle nature from nurture.
Association is not the same as causation
Many family factors are associated with an increase in the rate of one or
more child and adolescent psychiatric disorders. It is all too easy to fall
into the trap of assuming that association implies causation. If a family
characteristic (X) is associated with a child and adolescent psychiatric
disorder (Y), it could be the case that X causes Y, but two alternative
explanations also need to be considered: that Y is causing X, which is
known as reverse causality; and that both X and Y are due to the operation
of a third factor or confounder.
Reverse causality
It is plausible that a psychiatrically disturbed child or adolescent can influ-
ence family characteristics. For example, parental depression, anger, crit-
icism, coldness, over-protection, punitiveness or disengagement could all
be evoked by their child’s symptoms. Some of the most powerful evidence
for such effects comes from intervention studies. For example, one study
showed that when stimulant medication reduced a child’s ADHD, this
usually led to less maternal criticism of the child, more maternal warmth
towards the child, and an increase in the time that the mother spent
with the child (see Chapter 5). Of course, it could still be the case that
the parental negativity evoked by a child’s ADHD is also damaging to
that child’s development. The evidence suggests that there is often a
two-way effect: the children’s characteristics do indeed influence parental
behaviour, and the parental behaviour in turn has an independent effect
on the children.
Third factors
If a mother and daughter are both particularly fearful of spiders, it seems
natural to assume that the daughter has learned the anxiety from her
mother. An equally plausible alternative is that the mother’s fear and
the daughter’s fear have a common origin: perhaps mother and daughter
share a genetic tendency to be fearful, or perhaps they have both watched
the same horror film about spiders. Conceptually, adoption studies provide
the most straightforward evidence for genetic third factors. If arachno-
phobia were completely genetic, then adopted children would resemble
their biological parents but not their adoptive parents in this respect.
Other approaches are needed to identify environmental third factors. For
example, if having seen the same film accounts for parents and children
both fearing spiders, then an epidemiological approach should be able
to show that the association between parents’ fears and children’s fears
disappears once allowance has been made for the effect of film viewing.