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Pair bonding and hormones Dr N.E.Whitehead
Summary: This is another study in which there is a connection between a sexual behaviour and genetics, but as for other connections found previously, it is not strong, and many other factors would influence it including counselling. A study from the Karolinska Institute in Sweden shows that the strength of the bonding between men and women, cohabiting or married, depends on the amount of a particular genetic sequence which is connected with the hormone vasopressin, already shown to influence the bonding behaviour of voles. However the effect is only modest and the authors are quick to disclaim that the presence or absence of the favourable sequence represents the destiny of a relationship. Some four years ago a paper appeared in which the sexually promiscuous behaviour of meadow voles was profoundly influenced by manipulating a single gene, making them like relatively monogamous prairie voles(Lim et al. 2004). This gene was connected with the vasopressin receptor, a biochemical structure on cell membranes which detects the hormone. Vasopressin is a hormone connected with pair bonding but also has other important functions such as bodily water retention. Now an analogous study has been done on humans. So the natural question is: is the infidelity of spouses connected to their natural genetic constitution? Is infidelity inevitable? The study, done at the Karolinska Institute in Sweden (Walum et al. 2008) was part of a twin study and results from each twin were used. All participants were living with a partner, married or cohabiting. The exact gene structure of voles associated with vasopressin is not found in humans, so the equivalent was sought - again a vasopressin cell receptor (or detector). In humans it occurs in three different forms, only one of which turned out to be important, AVPR1a. Along with the genetic analysis the couples filled out questionnaires: have you divorced in the last year or had significant marital discord? AVPR1a is either absent, present once or present twice. Respectively, the strength of the pair bonding between the couples was: 48, 46.3, 45.5. So it decreased, one might say about 3 parts in 48 or 6%. Not very much, but perhaps pair bonding is not a very good measure. For the measure of whether there had been marital discord the results were 15%, 16% or 34% respectively. In other words marital discord more than doubled if two instances of the receptor were present. It also respectively increased the chances of cohabitation rather than marriage. The significance level for the association of pair bonding and instances of the receptor was p<0.01. So it was a real effect. Was it strong? This is classed statistically as a modest effect. Certainly there is no way one could do a genetic analysis before a relationship and then say “it will fail!”. It is in human nature to want to explain or explain away relationship discord as being “in the genes”. However it is a modest effect only. This is quite typical of what is found, and one might predict that future discoveries will also uncover only modest effects. This study was of heterosexuals only. It is interesting to speculate what might happen if the research were repeated with gay couples. Heterosexuals and homosexuals share the same genes, but violence among gay couples is several times that in straight couples. It may be coming from somewhere other than genes.
Lim, M.M., Wang, Z., Olazabal, D.E., Ren, X., Terwilliger, E.F. and Young, L.J. (2004) Enhanced partner preference in a promiscuous species by manipulating the expression of a single gene. Nature 429 754-757.
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