Genes cause more variation in verbal and speed of intelligence, and less in spatial. Environment has the most influence in spatial smarts.
Source: "Substantial Genetic Influence..." in the bibliography.
Swedish siblings
In June, Gerald McClearn of Penn State University and his colleagues reported a remarkable degree of genetic determination in the intelligence of Swedish twins. Inheritance -- genes -- accounted for 55 percent of the difference in ability in the population they studied, which included 110 pairs of identical twins 130 pairs of same-sex fraternal twins. All were more than 80 years old.
(Isn't this kind of bait and switch illegal? Can't help it... Even though everybody wants to know about quads, quints and septuplets, virtually all studies of multiple births have focused on twins. So we'll respond to the Iowa births by taking what we can get, and scoping out some recent twin research. And even though gobs of twin research focus on health and disease, we'll concentrate on stuff that supposedly distinguishes Homo sapiens from other animals -- intelligence and personality.)
Common sense might indicate that the genetic component would taper off as the twins aged and amassed more experience, McClearn says. Indeed he adds that many gerontologists "expected the genetic component to be reduced dramatically at this age. Yet in the group of Swedes that we studied, even the effects of over 80 years of environmental influence didn't eliminate the impact of heredity on cognitive ability."
Yet the results are not surprising in one sense, since intelligence is considered one of the most inheritable traits. Twenty percent of the variation in infants' intellectual ability has been attributabed to genes. The proportion rises to 60 percent in adults. In other words, genetic influence seems to increase with maturity.
Although McClearn did find that the percentage fell slightly -- to 55 percent in old age -- he says it's pointless to fixate on exact percentages. "None are meant to be precise. Nobody cares whether it's 50 percent or 60 percent." What's important is the general range, which comes down to about 50/50. For adults, he says, "The general literature converges on a number somewhere around this. Half of the variation of function is due to genetics, and half due to environment."
The genetic influence in the elderly Swedes did not reflect disease, he adds, since the twins were tested to eliminate dementias -- the category of impaired intellectual functioning that includes Alzheimer's disease -- before the tests.
The McClearn study, one of the first to examine specific components of intelligence, attributed 62 percent of processing speed, 52 percent of memory, but only 32 percent of spatial ability to genes. The study was not designed to explain the high influence of environment on spatial ability.
Not just ivory tower
Overall, McClearn says, the goal was not so much to pinpoint the genetic component of aging as to tease out the various effects of environment and genetics. "We are just as happy to find environmental influences as genetics." Ultimately, rather than examining intelligence in the abstract, the researchers, sponsored by the National Institute of Aging, were seeking ways to improve the functioning of older adults.
Since genes are a bit difficult to change, it may be encouraging to note that the McClearn study found that overall, the environment accounted for just under 50 percent in the variation in intelligence. The influence of "shared environment" -- being reared in the same family or working the same job -- was estimated at between 11 percent and 15 percent. The rest of the environmental variation was attributed to non-shared environment -- things like different jobs, housing, friends, illnesses or exposures to environmental toxins.
And because environment is controllable to some extent, it opens a path to improving the intellectual functioning of old people. McClearn says his group is still sifting through information on occupations, diet, exercise and other variables to explain some of the environmental differences -- which could point to helpful actions.
Still, genetics could also serve as a basis for action, since a gene associated with some cases of Alzheimer's disease has already been located. Figuring out what it does should give a picture of how that brain-destroying illness develops, and perhaps point to a treatment.
Let's get practical. Could genes explain why my kid brother's such a brat?
