consequence of inheriting these favorable genes. A better way to show that
parenting styles and other environmental variables have a causal effect on the
acquisition of intelligence would be to rear one group of children under one set
of environmental conditions and a comparable group under a different set of
conditions. Ideally, the children should be randomly assigned to the two con-
ditions, but random assignment is obviously socially and ethically impossible.
Still, some research comes close to performing the ideal experiment. Obser-
vations of children growing up in orphanages reveals that children who receive
loving affection from the caretakers will tend to average higher on IQ tests than
children who do not get the affection (Skeels, 1966). Other research has pro-
vided training to a group of low-income preschool children on the intellectual
skills necessary to do well in school, and has shown that such children improve
their IQ performance by an average of 10 to 15 points. Unfortunately, these
sorts of studies typically reveal that the gains are temporary. By the fourth
grade, the average IQ performance of the group that got the training declines to
the level of comparable children who did not receive the training (Bronfen-
brenner, 1974; Klaus & Gray, 1968; Ramey, Campbell, & Finkelstein, 1984).
However, if the intervention program is extended into the school years, evi-
dence suggests that the intervention has a beneficial effect on IQ performance
that extends beyond the first few years of school (Lazar, Darlington, Murray,
Royce, & Snipper, 1982; Miller & Bizzell, 1984).
A fairly dramatic environmental effect on IQ performance was accomplished
by Garber (1988), who placed a group of children who were previously labeled
to be at risk for mental retardation in an extensive home enrichment program.
Garber found that, by age 6, the group scored 30 points higher on an IQ test
than did a control group, and even by age 14 still scored about 10 points higher
than the control group. Another dramatic case is the Carolina Abecedarian
Project (Campbell & Ramey, 1994). In this project, infants from low-income
families were placed into intellectually enriched environments until they began
school. Compared with controls, the enriched children scored higher on tests of
intelligence, even 7 years after the end of the intervention.
Generational Environmental Changes: IQ Scores Are Rising One intriguing piece
of evidence for an environmental influence on IQ performance is the finding
that in this century there has been a steady worldwide rise in IQ scores (Flynn,
1984, 1987; see Neisser et al., 1996). The average gain has been about 3 IQ
points per decade. The result is that most intelligence tests have to be periodi-
cally restandardized in order to keep the mean equal to a score of 100. So peo-
ple who score 100 on an IQ test today (in 1997) would have averaged about 115
in 1947.
No one knows why IQ scores are rising. Among the proposed reasons (see
Neisser et al., 1996) is the idea that the world’s cultures are becoming in-
formationally more complex, because of television, urbanization, prolonged
schooling, and so on. Such complexity then produces improvements in the de-
velopment of intellectual skill. Another idea is that the IQ increases are due to
nutritional improvements, perhaps the same improvements that have also led
to nutritionally based increases in height. Whatever the reason, it must be
something in the environment that is producing the rising IQ scores. Certainly
798 R. Kim Guenther