In summary, genetic versus environmental
influences are not either/or choices. Clearly, both
play some role in the development and expression
of intelligence. Behavioral geneticists do not claim
that environment plays no role in IQ; rather, their
data clearly implicate environmental factors and
should serve as a stimulus for more research on the
nature and effect of these factors on the development
of intelligence (Deary et al., 2010; Petrill, 2005).
Stability of IQ Scores and the“Flynn”effect.
As indicated earlier, one method of assessing the reli-
ability of a measure is by computing a test–retest cor-
relation. This gives us a sense of how stable scores are
over time. As noted by Schuerger and Witt (1989),
IQ scores tend to be less stable for young children
andmorestableforadults.Thisalsofitswellwiththe
notion that IQ scores are more influenced by the
environment (e.g., school) at younger rather than
older ages, Further, and not too surprisingly, a longer
test–retest interval (e.g., 10 years vs. 1 year) results in
lower reliability/stability estimates. Figure 7-3 depicts
stability of IQ scores as a function of age.
The implication of these findings is clear. Clin-
icians cannot assume that a single IQ test score will
accurately characterize an individual’s level of intel-
ligence throughout his or her life span. IQ scores do
tend to change, and this is especially true for young
children. For this reason, clinicians often describe
the individual’s“present level of intellectual func-
tioning”in their test reports. A variety of influences
(e.g., illness or motivational and emotional changes)
may affect an individual’s score.
Finally, it is worth briefly introducing theFlynn
effect (Flynn, 1984; Flynn & Weiss, 2007). This
refers to the empirical finding that from 1972 on,
Americans’IQ scores have on average increased
3 points each decade. There are a number of possi-
ble explanations for this effect, ranging from people
are getting smarter to contemporary humans are
exposed more to IQ tests or at least similar types
of cognitive tasks in everyday life (Hunt, 2010).
Whether the Flynn effect reflects better educational
outcome or not, clinicians must be sure to use the
most recent normative data to ensure appropriate
comparisons to one’s peers and cohort.The Clinical Assessment of Intelligence
In this section, we briefly describe several of the
most frequently used intelligence tests for children
and adults.1.0
0.9
0.8
0.7
0.6
3 15 27 39 51
Age (years)
Increase in reliability by ageF I G U R E 7-3 Graph depicting the relationship between age and stability.
SOURCE: Scheurger and Witt,“The temporal stability and individually tested intelligence,”Journal of Clinicial PsychologyVol. 45, p. 300. Copyright © 1989 by
John Wiley and Sons, Inc. Reprinted by permission.
THE ASSESSMENT OF INTELLIGENCE 205