Environmental factors
No one argues for genetics alone in shaping behavior. Lewontin, Rose, and Kamin (1984) “cannot think of any significant human social behavior that is built into our genes in such a way that it cannot be shaped by social conditions.” Environment includes a broad array of effects on intelligence; some influence whole populations while others contribute to individual differences. These influences include biological as well as social and cultural factors.
Biological factors such as malnutrition, exposure to toxic substances, prenatal and perinatal stressors may result in lowered intelligence. Prolonged malnutrition during childhood has negative long-term intellectual effects, but interestingly, in a study on prenatal malnutrition, Stenin, Susser, Saenger, and Marolla studied the test scores of 19-year-old Dutch males born during a wartime famine (Neisser et al., 1996), and found that exposure to famine had no effect on adult intelligence. Exposure to lead can have a negative effect on intelligence: Neisser et al. (1996) administered IQ tests to children with high blood lead levels throughout childhood, and found they scored substantially lower. Fetal alcohol syndrome, due to prenatal exposure to excessive amounts of alcohol, may lead to mental retardation. Even smaller amounts of alcohol may harm: mothers who reported drinking 1.5 oz of alcohol daily during pregnancy had children who scored an average of 5 points lower on IQ tests by age four (Neisser et al., 1996). Perinatal factors such as complications during delivery may also influence intelligence. The correlation between very low birth weight (less than 2,500 gm.) and later intelligence is fairly large (Breslau et al., 1994, Baumeister & Bacharach, 1996, as cited in Mackintosh, 1998).
Social and cultural aspects of environment may influence intelligence: Werner and Smith (1992) found that temperament affected developmental outcomes for children, beginning at birth. Infants who smiled often and were affectionate attracted more care and emotional support from their parents and others. Schools promote the development of intellectual skills such as systematic problem-solving, abstract thinking, and categorization; children who attend regularly may be expected to benefit more than those who attend sporadically. Plomin (1991) suggests other factors such as parental affection, birth order, gender differences, experiences outside the family, accidents, and illnesses may account for differences in IQ between siblings. In their study of adoptive and biologically related families with children between 16 and 22 years of age, Scarr and Weinberg (1983) found environment more powerful in influencing IQ level in the young child than the young adult. They argue that “by providing better schooling, nutrition, health care, psychological services and the like, we can raise the level of intellectual development.”
In the short run, a number of interventions have been shown to raise test scores. The Venezuelan Intelligence Project involved exposing hundreds of seventh-grade children from underprivileged backgrounds to a specialized curriculum focused on thinking skills (Chipuer et al., 1990), and produced considerable improvements on a wide range of tests. Another program, Head Start, is specifically geared to underprivileged children around four years old and is able to document an improvement in test scores over the length of the program. These effects dissipate over time, however, and by the end of grade school, there are no significant differences for Head Start participants over other children from similar environments (Darlington, et al., 1980). However, follow-up studies such as the Perry Preschool study (Schweinhart & Weikart, 1988) indicate that children who participated in such programs as preschoolers are less likely to be placed in special education classes, less likely to be held back in grade and more likely to finish high school.
A change in a single gene may change the structure of an organism, but Goodwin (1994) argues that we cannot logically step from this to the idea that genes contain all the information needed to create the structure. Viewing organisms as the sum of their genes is reductionistic: organisms must be studied “as dynamic systems with distinctive properties that characterize the living state” (Goodwin). Development of an organism is an epigenetic process: at every step of development, the next step depends on the organism’s current biological state, which is a function of both genetics and environment. Organisms construct their environments, change them, interpret sensory experiences, and change the pattern of variation (Lewontin, Rose, & Kamin, 1984). In humans, mental states affect environments through conscious action; the relationship between human and environment is a dual development of each. Kaye (1992) suggests that “the question of human nature is not simply a biological one, no matter how many genetic correlates of character are discovered.” Genes, environment, and the interaction between them all contribute to influence the development of individual differences in the complex phenomenon known as “intelligence.”
Bibliography
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