On the Fifth day of Christmas tradition demands that you send your true love Five Gold Rings. Even with a small circle of readers, this could prove expensive. The next best thing is to send five special findings regarding genetics and intelligence differences, as an end of year gift.
(1) The heritability of intelligence increases from about 20% in infancy to perhaps 80% in later adulthood. This seems to be due to a process of genetic magnification, perhaps through the creation of personal micro-environments.
(2) Intelligence captures genetic effects on diverse cognitive and learning abilities, which correlate phenotypically about 0.30 on average but correlate genetically
about 0.60 or higher. (Genetic correlations estimate the extent to which genetic effects on one trait are correlated with genetic effects on another trait independently of the heritabilities of the two traits. They can be thought about roughly as the probability that genes associated with one trait are also associated with the other trait. Genetic correlations are derived from the genetic analysis of covariance between traits using the same quantitative genetic methods used to analyse variance.) Multivariate genetic research—both from twin studies and GCTA—suggests that most of the genetic action is general across diverse cognitive abilities rather than specific to each ability. Intelligence is a good target for gene-hunting
because it indexes these generalist genes.
(3) Assortative mating is greater for intelligence (spouse correlations ~0.40) than for other behavioural traits such as personality and psychopathology (~0.10) or physical traits such as height and weight (~0.20). Assortative mating pumps additive genetic variance into the population every generation, contributing to the high narrow heritability (additive genetic variance) of intelligence. Verbal intelligence shows greater assortative mating (~0.50) than nonverbal intelligence (~0.30), perhaps because it is easier to gauge someone’s verbal ability such as vocabulary than their nonverbal intelligence such as spatial ability. Assortative mating for intelligence is caused by initial selection of a mate (assortment) rather than by couples becoming more similar to each other after living together (convergence). In part, spouses select each other for intelligence on the basis of education—spouses correlate about 0.60 for years of education19—which correlates about 0.45 with intelligence. Assortative mating may be greater than it is for intelligence for a few other traits such as social attitudes, smoking and drinking, although these traits might be affected by
convergence. Assortative mating increases additive genetic variance in that the offspring differ more from the average than they would if mating were random. The increase in additive genetic variance can be substantial because its effects accumulate generation after generation until an equilibrium is reached. For example, if the heritability of intelligence with random mating were 0.40, the additive genetic variance of intelligence would increase by one-quarter at equilibrium given assortative mating of 0.40, Falconer and MacKay (1996) equation 5, Table 10.6, p. 176.
(4) Unlike psychiatric disorders, intelligence is normally distributed with a positive end of exceptional performance that is a model for ‘positive genetics’. Mind you, although the authors do not say so, some people are probably extremely positive in their behaviours: calm and kind and helpful. It is simply that no-one has particularly looked for them, because they are not a nuisance and do not call attention to themselves.
(5) Intelligence is associated with education and social class and broadens the causal perspectives on how these three inter-correlated variables contribute to social mobility, and health, illness and mortality differences. Strange as it may seem, social class and education are also heritable.
The authors propose that three particular sets of findings are so common in genetic research that they could almost be considered laws: all traits are heritable, all traits show environmental effects, and the heritability of traits is brought about by many genes of small effects. Remembering those three observation will put you well ahead of most commentators when discussing the genetics of intelligence.
The authors describe the new technique of Genome-wide Complex Trait Analysis thus:
However, instead of using genetic similarity from groups differing markedly in genetic similarity such as monozygotic and dizygotic twins, GCTA uses genetic similarity for each pair of unrelated individuals based on that pair’s overall similarity across hundreds of thousands of single nucleotide polymorphisms (SNPs) for thousands of individuals; each pair’s genetic similarity is then used to predict their phenotypic similarity. Even remotely related pairs of individuals (genetic similarity greater than 0.025, which represents fifth-degree relatives) are excluded so that chance genetic similarity is used as a random effect in a linear mixed model. The power of the method comes from comparing not just two groups like monozygotic and dizygotic twins, but from the millions of pair-by-pair comparisons in samples of thousands of individuals. In contrast to the twin design, which only requires a few hundred pairs of twins to estimate moderate heritability, GCTA requires samples of thousands of individuals because the method attempts to extract a small signal of genetic similarity from the noise of hundreds of thousands of SNPs.
In my non-genetic language, this is like hunting through thousands of coded messages in order to find similarities between messages by looking at clumps of individual variations within the common code. Its great power, to me, is that it does not depend the specific instance of twins (seen as unrepresentative by critics of genetics) but on relative similarity on a very broad dimension of genetic relatedness.
Have a look at this excellent expert review: Genetics and intelligence differences: five special findings. R Plomin and IJ Deary. Molecular Psychiatry (2014), 1–11
The review is written by the two persons in the world best able to explain modern research in genetics and intelligence. It is an excellent summary of the current state of play in the genetics of intelligence. Although most science research is incremental, and scientific “breakthroughs” happen mostly in news rooms when there is nothing much else to print, it might be that 2015 will provide a significant increase in understanding how a genome builds the problem solving brain. Although no one can accurately determine the size of the task, and it is generally thought a very complex problem, sample sizes are getting much bigger, genomic chips cheaper and more powerful, and the raw data processing power needed to interpret the results is increasing steadily.
Happy New Year!