Why are some siblings so similar and some so different? Identical twins, who share all their genes, are almost invariably more similar than fraternal twins, who are no more closely related than any other full siblings. And half-siblings, who only share one parent, are almost always more different still.
Comparison of full with half siblings can reveal the different contributions of each parent, and can be used to test the imprinted brain theory because it claims that mothers and fathers make different contributions to the cognitive configuration of a child. The theory also proposes that human beings have evolved two parallel modes of cognition: mechanistic cognition in relation to the physical world of objects and natural phenomena, and mentalistic cognition in relation to the psychological world of people and mental phenomena. Furthermore, it suggests that because males are averagely more mechanistic and females more mentalistic, the father’s genes favor mechanistic cognition by contrast to the mother’s, which favor mentalism.
Unfortunately though, measures of these parallel modes of cognition do not as yet exist, and cognitive science is dominated in this respect by an existing, but somewhat different distinction, that between empathizing and systemizing.
Nevertheless, a study by Gillian Ragsdale and Robert Foley tests one of its predictions by way of examining parent-of-origin effects on empathy and systemizing as proxies for mentalistic and mechanistic cognition. Such effects can be gauged by assessing the mean amount of DNA that different kinds of siblings receive on particular chromosomes from each parent.
Chromosomes are long loops of DNA on which genes are strung. They are of two types: autosomes, of which there are 22 in the human genome, and sex chromosomes, which can either be an X or a Y. Each parent normally bequeaths 22 autosomes and a single sex chromosome to their offspring. The complication is that females get an X sex chromosome from each parent, but males a Y from the father and an X from the mother.
A further complication is that we now know that some critical genes are imprinted, and only expressed when inherited from one parent or the other. Although such genes are inherited from both parents, the fact that they are only expressed from one copy means that they can be distinguished as being either paternal of maternal.
As the table below shows, full sisters share all the DNA on the X chromosome they got from their fathers and half of all the rest. Full brothers, by contrast, lack a paternal X, but share half of all their other DNA, as they also do with their full sisters. Paternal half-sisters, who share the same father but different mothers, have identical paternal X DNA but no maternal X or autosomal DNA in common. However, they share half their paternal autosomal DNA, as do paternal half-brothers and sisters. Maternal half sisters are the other way round: sharing half their maternal autosomal and X chromosome DNA, and maternal half-brothers and sisters are the same.
Ragsdale and Foley studied 99 pairs of full siblings, 46 pairs of maternal half-siblings, and 23 pairs of paternal half-siblings. They found that males scored significantly higher on systemizing but that female systemizing was decreased, as previous studies have suggested. The researchers also report that their results support “a model of differential imprinting influences on empathy such that weak empathy is paternally influenced and strong empathy is equally or maternally influenced,” much as the imprinted brain theory predicts.
Nevertheless—and seemingly contrary to the theory—they also found that there is “support for a maternal influence on systemizing,” and go on to note that their results “suggest that empathy and systemizing do not lie at opposite ends of the same spectrum with regard to the influence of autosomal imprinted genes.” Furthermore, they comment that “a maternal influence on systemizing is consistent with evidence that the skills associated with strong systemizing (taken to overlap with ‘mechanistic thinking’) such as tool making, mathematics, physics, computing and engineering depend on the higher cognitive functions of the generally maternally influenced prefrontal cortex…”
However, the authors also point out that an alternative explanation for the apparent trade-off between empathy and systemizing is that there are additional sex-specific influences, and they cite evidence like that mentioned by me in a previous post linking empathy and social cognition to X chromosome genes.
They conclude that “The apparent tradeoffs between mentalistic and mechanistic cognition, and similarly between empathy and systemizing, may be due to sex-specific influences that differentiate between sons and daughters, producing the well-observed sexual dimorphism in these traits.” This is exactly what the imprinted brain theory proposes, and as the authors also point out, “A larger scale study might better differentiate between alternatives such as the influence of X-linked imprinted genes and sex-specific environmental or genetic factors.”
I agree entirely, and my personal guess is that both X-linked genes and sex-specific imprinted autosomal genes expressed in the cortex will ultimately be found to explain the findings. Time will tell, but Ragsdale and Foley are to be congratulated for starting the clock ticking.
(With thanks and acknowledgements to Gillian Ragsdale.)