actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high. Since heritability increases during childhood and adolescence, and even increases greatly between 16 and 20 years of age and adulthood, one should be cautious drawing conclusions regarding the role of genetics and environment from studies where the participants are not followed until they are adults. Furthermore, there may be differences regarding the effects on the g-factor and on non-g factors, with g possibly being harder to affect and environmental interventions disproportionately affecting non-g factors. Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also regression toward the mean. Estimates Various studies have estimated the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United States. It has been found that estimates of heritability increase as individuals age. Heritability estimates in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. The brain undergoes morphological changes in development which suggests that age-related physical changes could contribute to this effect. A 1994 article in Behavior Genetics based on a study of Swedish monozygotic and dizygotic twins found the heritability of the sample to be as high as 0.80 in general cognitive ability; however, it also varies by trait, with 0.60 for verbal tests, 0.50 for spatial and speed-of-processing tests, and 0.40 for memory tests. In contrast, studies of other populations estimate an average heritability of 0.50 for general cognitive ability. In 2006, David Kirp, writing in The New York Times Magazine, summarized a century's worth of research as follows, "about three-quarters of I.Q. differences between individuals are attributable to heredity." Shared