Intelligence runs in families. But why? Are our intellectual abilities mostly inherited? Or are they molded by our environment?
Few issues arouse such passion or have such serious political implications. Consider: If we mainly inherit our differing mental abilities, and if success reflects those abilities, then people’s socioeconomic standing will correspond to their inborn differences. This could lead to those on top believing their intellectual birthright justifies their social positions.
But if mental abilities are primarily nurtured by our environments, then children from disadvantaged environments can expect to lead disadvantaged lives. In this case, people’s standing will result from their unequal opportunities.
For now, as best we can, let’s set aside such political implications and examine the evidence.
Do people who share the same genes also share mental abilities? As you can see from FIGURE 63.1, which summarizes many studies, the answer is clearly Yes. Consider:
Other evidence points to the effects of environment. Twin studies show some environmental contribution to IQ score variation among top scorers (Brant et al., 2009; Kirkpatrick et al., 2009). Where environments vary widely, as they do among children of less-educated parents, environmental differences are more predictive of intelligence scores (Rowe et al., 1999; Tucker-Drob et al., 2011; Turkheimer et al., 2003). Studies also show that adoption enhances the intelligence scores of mistreated or neglected children (van IJzendoorn & Juffer, 2005, 2006).
Seeking to disentangle genes and environment, researchers have compared the intelligence test scores of adopted children with those of (a) their adoptive siblings, (b) their biological parents (the providers of their genes), and (c) their adoptive parents, the providers of their home environment. During childhood, the intelligence test scores of adoptive siblings correlate modestly. Over time, adopted children accumulate experience in their differing adoptive families. So would you expect the family-environment effect to grow with age and the genetic-legacy effect to shrink?
If you would, behavior geneticists have a stunning surprise for you. Mental similarities between adopted children and their adoptive families wane with age, until the correlation approaches zero by adulthood (McGue et al., 1993). Genetic influences – not environmental ones – become more apparent as we accumulate life experience. Identical twins’ similarities, for example, continue or increase into their eighties. Thus, report Ian Deary and his colleagues (2009), the heritability of general intelligence increases from “about 30 percent” in early childhood to “well over 50 percent in adulthood.” In one massive study of 11,000 twin pairs in four countries, the heritability of g increased from 41 percent in middle childhood to 55 percent in adolescence to 66 percent in young adulthood (Haworth et al., 2010). Similarly, adopted children’s verbal ability scores over time become more like those of their biological parents (FIGURE 63.2). Who would have guessed?
Genes make a difference. Even if we were all raised in the same intellectually stimulating environment, we would have differing aptitudes. But life experiences also matter. Human environments are rarely as impoverished as the dark and barren cages inhabited by deprived rats that develop thinner-than-normal brain cortexes (see Module 50). Yet severe deprivation also leaves footprints on the human brain.
Nowhere is the intertwining of biology and experience more apparent than in impoverished human environments such as J. McVicker Hunt (1982) observed in a destitute Iranian orphanage. The typical child Hunt observed there could not sit up unassisted at age 2 or walk at age 4. The little care the infants received was not in response to their crying, cooing, or other behaviors, so the children developed little sense of personal control over their environment. They were instead becoming passive” glum lumps.” Extreme deprivation was bludgeoning native intelligence – a finding confirmed by other studies of children raised in poorly run orphanages in Romania and elsewhere (Nelson, et al., 2009; van IJzendoorn et al., 2008).
Aware of both the dramatic effects of early experiences and the impact of early intervention, Hunt began a program of tutored human enrichment. He trained caregivers to play language-fostering games with 11 infants, imitating the babies’ babbling, then engaging them in vocal follow-the-leader, and finally teaching them sounds from the Persian language. The results were dramatic. By 22 months of age, the infants could name more than 50 objects and body parts, and so charmed visitors that most were adopted – an unprecedented success for the orphanage.
Hunt’s findings are an extreme case of a more general finding: Among those economically impoverished, environmental conditions can depress cognitive development. Schools with many poverty-level children often have less-qualified teachers, as one study of 1450 Virginia schools found. So these children may receive a less-enriched education. And even after controlling for poverty, having less-qualified teachers predicted lower achievement scores (Tuerk, 2005). Malnutrition also plays a role. Relieve infant malnutrition with nutritional supplements, and poverty’s effect on physical and cognitive development lessens (Brown & Pollitt, 1996).
Do studies of such early interventions indicate that providing an “enriched” environment can “give your child a superior intellect,” as some popular products claim? Most experts are doubtful (Bruer, 1999). Although malnutrition, sensory deprivation, and social isolation can retard normal brain development, there is no environmental recipe for fast-forwarding a normal infant into a genius. All babies should have normal exposure to sights, sounds, and speech. Beyond that, Sandra Scan’s (1984) verdict still is widely shared: “Parents who are very concerned about providing special educational lessons for their babies are wasting their time.”
Still, explorations of intelligence promotion continue. Some parents, after exposing their 12- to 18-month-old babies to educational DVDs such as from the Baby Einstein series, have observed their baby’s vocabulary growing. To see whether such cognitive growth is a result of the DVD exposure, or simply of infants’ natural language explosion, two research teams assigned babies to DVD exposure or a control group (DeLoache et al., 2010; Reichert et al., 2010). Their common finding: The two groups’ word-learning did not differ.
Later in childhood, schooling is one intervention that pays intelligence score dividends. Schooling and intelligence interact, and both enhance later income (Ceci & Williams, 1997, 2009). Hunt was a strong believer in the ability of education to boost children’s chances for success by developing their cognitive and social skills. Indeed, his 1961 book, Intelligence and Experience, helped launch Project Head Start in 1965, a U.S. government-funded preschool program that serves more than 900,000 children, most of whom come from families below the poverty level (Head Start, 2010). Does it succeed? Generally, the aptitude benefits dissipate over time (reminding us that life experience after Head Start matters, too). Psychologist Edward Zigler, the program’s first director, nevertheless believed there are long-term benefits (Ripple & Zigler, 2003; Zigler & Styfco, 2001) .
Genes and experience together weave the intelligence fabric. (Recall from Module 14 that epigenetics is one field that studies this nature-nurture meeting place.) But what we accomplish with our intelligence depends also on our own beliefs and motivation. One analysis of 72,431 collegians found that study motivation and study skills rivaled previous grades and aptitude as predictors of academic achievement (Crede & Kunce!, 2008). Motivation even affects intelligence test performance. Four dozen studies show that, when promised money for doing well, adolescents score higher (Duckworth et al., 2011) .
Psychologist Carol Dweck (2006, 2007, 2008) reports that believing intelligence is biologically set and unchanging can lead to a “fixed mindset.” Believing intelligence is changeable, a “growth mindset” results in a focus on learning and growing. As collegians, these believers also tend to happily flourish (Howell, 2009). Dweck has developed interventions that effectively teach young teens that the brain is like a muscle that grows stronger with use as neuron connections grow. Indeed, as we noted earlier, superior achievements in fields from sports to science to music arise from disciplined effort and sustained practice (Ericsson et al., 2007).