The Civic Platform - A Political Journal of Ideas and Analysis

By Jack Kaplan, Reply by Frank J. Sulloway
In response to Parallel Lives (November 30, 2006)

To the Editors:

Frank Sulloway writes ["Parallel Lives," NYR, November 30, 2006] that “genes are responsible for…about 60 percent of individual differences in general intelligence. ” Although this claim is frequently made by researchers in the field of behavioral genetics, I do not believe it stands up to careful scrutiny.

Estimates of the proportion of variability in general intelligence that is explained by genetics are based on three types of research—studies of identical twins raised apart, studies of biologically unrelated siblings raised in the same home, and studies comparing identical to fraternal twins. Each of these types of studies is based on one or more highly questionable assumptions.

[I have snipped the substantive part of Prof. Kaplan's letter to shorten this post, but mostly because I believe Frank Sulloway incorporates Kaplan's criticisms in his rejoinder. Members are invited to read Jack Kaplan's full letter on the NTR website-TOM ]

Jack Kaplan
Professor of Statistics
Quinnipiac University
Hamden, Connecticut

Frank J. Sulloway replies:
Professor Kaplan has criticized several key assumptions that underlie heritability estimates for IQ. He seems unaware, however, that these issues have all been addressed empirically during the last two decades and are no longer regarded as “highly questionable. ” For example, Thomas Bouchard and colleagues, in 1990, analyzed 56 sets of identical twins reared apart and later reunited in adulthood.[1] What is particularly instructive about Bouchard et al.’s study is their extensive analysis of data on the rearing environments of the twins included in their sample. Bouchard’s team assessed nine different aspects of the twins’ rearing environments, including the adoptive parents’ levels of education, the adoptive father’s socioeconomic status, parental treatment, and various cultural and intellectual resources available within the home.

Bouchard et al. found that the rearing environments of the twins were indeed moderately correlated (r = .22), confirming a potential bias in the estimation of heritability. Bouchard’s team then assessed the direct influence of each of these environmental factors on IQ, from which they were able to determine the total contribution to heritability. The maximum contribution for any one of these nine individual environmental measures turned out to be a minuscule .03, and the mean contribution for the nine measures was just .006. In other words, the heritability estimate for this set of reared-apart twins, which was .69, would be reduced to about .66 based on environmental similarities among the adoptive homes of the twins.

Bouchard and his colleagues also tested the possible contribution made to IQ by pre-separation and post-reunion contact between the twins, another of the key issues raised by Professor Kaplan. Total contact time between the twins averaged 5.1 months prior to separation and 20.3 months following first reunion, when the twins were mostly in their late twenties and early thirties. Degree of contact accounted for almost none of the similarity in the twins’ IQs (r = .14, which was not significantly different from zero).

The last of the questionable assumptions about heritability estimates involves the issue of whether identical twins experience environments that are more similar than do fraternal twins. This issue is important because heritability is sometimes calculated as twice the difference between the correlations for IQ scores among identical as opposed to fraternal twins. Behavioral geneticists themselves have long acknowledged the potentially problematic nature of this method, although some research suggests that fraternal twins are not substantially different from identical twins in this regard. In any event, an obvious way around this potential confounding influence is to compare the IQ scores of identical twins reared apart with those of fraternal twins reared apart. With this approach, a shared “twin environment” is no longer a confounding factor for either class of twins.

Five previous studies of identical twins reared apart have yielded a mean-weighted heritability estimate of .75 (based on 158 twin pairs)—that is, 75 percent of individual differences in IQ scores are accounted for by heredity.[2] Although conducted in different countries and time periods, and with different measurement instruments, these results have been remarkably consistent (.64 to .78). Two previous studies of fraternal twins reared apart have yielded a mean-weighted heritability estimate of .38 (based on 73 twin pairs). These collective data yield a heritability estimate—unconfounde d by shared twin environments— of .74 (.75 minus .38, multiplied by 2). It is worth noting that these 7 studies of twins reared apart actually yield a higher heritability estimate than do the 75 studies involving identical and fraternal twins reared together, which is only .52 (based on 4,672 identical and 5,546 fraternal twin pairs).[3] The difference between these two heritability estimates is likely due to the relatively younger age of the participants in studies where twins have been raised together, as heritability estimates increase with age.

In sum, for twins, heritability estimates for IQ appear to be between .50 and .70, depending on the particular method by which IQ is calculated, the age of the study participants, and measurement error. The possible confounding influences mentioned by Professor Kaplan appear to make almost no difference in any of these findings. This general conclusion does not mean that environmental influences on IQ are unimportant. On the contrary, abundant evidence has shown that family environments make a substantial contribution to intelligence, especially before children reach adulthood and especially in impoverished environments that do not allow for the full development of genetic predispositions.[4] The increasingly evident portrait of human development that has emerged from these twin studies is one of nature via nurture, as people growing up are drawn to environments that provide the best outlets for their inborn dispositions and abilities.

Notes
[1] Thomas J. Bouchard, Jr., David T. Lykken, Matthew McGue, Nancy L. Segal, and Auke Tellegen, “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart,” Science, Vol. 250 (1990), pp. 223–228.

[2] Nancy L. Segal, Entwined Lives: Twins and What They Tell Us about Human Behavior (Dutton, 1999), pp. 135–136.

[3] Thomas J. Bouchard, Jr., and Matthew McGue, “Familial Studies of Intelligence: A Review,” Science, Vol. 212 (1981), pp. 1055–1059.

[4] Kathleen McCartney, Monica J. Harris, and Frank Bernieri, “Growing Up and Growing Apart: A Developmental Meta-Analysis of Twin Studies,” Psychological Bulletin, Vol. 107 (1990), pp. 226–237; Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D’Onofrio, and Irving I. Gottesman, “Socioeconomic Status Modifies Heritability of IQ in Young Children,” Psychological Science, Vol. 14 (2003), pp. 623–628.

The New York Review of Books
March 15, 2007
http://www.nybooks. com/articles/ 19985