Chapter 1 What Is Psychology? 25not establish causation. It is often tempting to as-
sume that if variable A predicts variable B, then
A must be causing B, but that is not necessarily
so. A positive correlation has been found be-
tween the number of hours that children watch
television between ages 1 and 3 and their risk
of hyperactivity (impulsivity, attention problems,
difficulty concentrating) by age 7 (Christakis et
al., 2004). Does this mean that watching TV
causes hyperactivity? Maybe, but it is also possible
that children with a disposition to become hyper-
active are more attracted to television than those
disposed to being calm. Or perhaps the harried
parents of distractible children are more likely
than other parents to rely on TV as a babysit-
ter. It is also possible that neither variable causes
the other directly: Perhaps parents who allow
their young kids to watch a lot of TV have atten-
tion problems themselves, and therefore create
a home environment that fosters hyperactivity
and inattentiveness. Likewise, that negative cor-
relation we mentioned between TV watching and
grades might exist because heavy TV watchers
have less time to study, or because they have some
personality trait that causes an attraction to TV
and an aversion to studying, or because they use
TV as an escape when their grades are low... you
get the idea.
The moral: When two variables are associ-
ated, one variable may or may not be causing the
other.
Explore the Concept Correlation vs. Causation
at mypsychlabCautions About Correlations LO 1.15
Correlational studies are common in psychology
and often make the news. But beware; many sup-
posed correlations reported in the media or on the
Internet are based on rumor and anecdote instead
of reliable evidence. Some are based on mere co-
incidence and are meaningless, and are therefore
called illusory correlations.
The alleged link between vaccines and autism
is an illusory correlation, probably a result of the
fact that most symptoms of childhood autism
emerge at about the same time that children are
vaccinated. Some parents think the culprit is thi-
merosal, a preservative that was used in childhood
vaccines until 1999 and is now contained in trace
amounts in only a few. Yet there is no convincing
evidence that thimerosal ever was involved in au-
tism, and after it was removed from most vaccines,
the incidence of autism did not decline, as it would
have if thimerosal were to blame. Moreover, study
after study has failed to find any causal connec-
tion whatsoever between vaccination and autism
(Mnookin, 2011; Offit, 2008). In one major study
of all the children born in Denmark between 1991
and 1998 (more than a half million children), the
incidence of autism in vaccinated children was
actually a bit lower than in unvaccinated children
(Madsen et al., 2002). Unfortunately, rates of mea-
sles and whooping cough, which can be fatal, are
rising in children whose frightened parents have
refused to have them vaccinated.
Even when correlations are meaningful, they
can be hard to interpret because a correlation does
The number of hours toddlers spend watching TV is correlated with their risk of being hyperactive a few years later.
Does that mean TV watching causes hyperactivity problems? What other explanations for this correlation are possible?