Humans in Love

Sep 23 2008 Published by under Behavioral Neuro
A good while ago I did a Friday Weird Science which I thought was really cool. Unfortunately, it just wasn't...weird...enough, and so I put it into poetry, because everything is better in verse. It was on Prairie Voles and monogamy, and was called Prairie Voles in Love:

Out on the lonely prairie, gazing at the stars above
I saw through the night
the wondrous sight
Of prairie voles in love

So you can imagine my happiness when I found out that it's not just in voles!!! A study came out recently assessing changes in vasopressin in humans!
Walum et al. "Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans." PNAS, 105(37), 2008.

I realize that I'm a little behind the times, it came out on Sept. 16th, after all. It's SO last week. But I thought it was cool, and wanted to read the whole paper for myself. And I DO have a Journal Club presentation coming up. I figured I can run the various options past ya'll, and maybe you can let me know which one you think is the most interesting.
As some of you may already be aware, monogamy is not really widespread among species. There are lots of variations on mate choice, from harems with many females and one male (like zebras), or polyandry, where one female has many males with which she mates, and they all hope some of the kids are theirs (like bees). But true monogamy is pretty rare, and found generally in only about 3% of mammals, and even more rarely among birds or insects.
Most mammals which exhibit monogamy are among the primates, but there is one very important rodent, the prairie vole.
Extensive work in voles has found that prairie voles are truly monogamous, while their very similar subspecies, meadow voles, are a buncha hos. The two species are almost genetically identical, and have overlapping habitats, yet almost never interbreed, precisely because of this. A prairie vole wants someone to grow old with, and a meadow vole wants fresh meat. It turns out that the difference between a faithful male vole and a player comes down to one hormone: vasopressin (AVP). AVP is a hormone produced in the posterior pituitary, which also is known for regulating things like kidney function and blood pressure. In male voles, vasopressin appears to also regulate those little warm fuzzy feelings they get from their spouse. They have higher levels of AVP receptors in certain brain areas. Even better, researchers found that you could make commitment-phobic voles monogamous by increasing either AVP or the numbers of AVP receptors in male voles.
So then everyone started asking: what about humans? Is there something similar in humans that can regulate monogamy? We have a vaspressin receptor (AVPR1A), and though the sequences don't tally perfectly with those found in prairie voles, there is a repeat sequence that is polymorphic, which means there are several gene possibilities. Additionally, AVPR1A receptor changes have been foun din some patients with autism or other social interaction disorders, which implies that AVP in male humans can have effects on social interactions. So a large group in Sweden started looking at the differences in human AVPR1A genes in a large population of male twins, all of whom were married or in long-term committed relationships.
What they found was that there was a significant difference in one polymorphism, the RS3 (producing an allele known as 334), and people's scores on a Partner Bonding Scale. The Partner Bonding Scale is a test that looks at human behaviors that correspond to behavioral patterns of pair bonding in other mammals. Men who carried double copies of the 334 allele scored lower on the Partner Bonding Scale than those who did not. Furthermore, men with 334 reported more marital problems in the Social Readjustment Rating Scale, particularly on questions relating to marital crisis and threat of divorce. While only 15% of men with one or no 334 allele had experienced threat of divorce within the past year, 34% of 334-expressing men had. Not only that, a double 334 allele was associated with the existence of marital status in the first place, with 334 men more likely to be living with a partner who they were not married to. And men who were not married or in a partnership scored even lower on the Pair Bonding Assesment, which lead to the hypothesis that the 334 gene was associated with avoidance of or problems with marital status.
Finally, they asked the men's wives or partners what they thought. The wives of men with two 334 alleles reported less expression of affection, and less cohesion than women who were married to men with no 334. So the general conclusion that they came to is that the AVPR1A polymorphism might contribute to differences in male pair bonding in humans the same way it works in voles. The effect size that they got for a 334 allele and Pair Bonding was very similar to effect sizes found for the dopamine D4 receptor and novelty seeking, as well as serotonin transporter polymorphisms and neurotic traits, which means that there is a little more likelihood of this being a real effect.
But I wouldn't race out and get your Significant Other genotyped just yet. After all, there's more than one genetic factor contributing to pair bonding, not to mention the huge number of environmental factors also involved. Humans have a much more complex societal structure than voles, and societal expectations may make a very large difference both in whether males enter in to pair bonds and how well they do in them. And there are many other questions as well. Are men with two 334 alleles more likely to choose or form pair bonds with relatively incompatible people, or people with different expectations of partnership? Are men with no 334 alleles simply people who back down from an argument and avoid conflict? Clearly all of these variations must be all right, as they persist in roughly equal distributions in the sample. So perhaps the 334 is good for something else. Of course genetic involvement doesn't take into account things like societal expectations. 334 allele or not, the environment you grow up in has a lot to do with your perceived role in society later on, whether you feel you have to get married and what acceptable marriages are like.
And ladies, for all we know some of it might be us! In female prairie voles, it is oxytocin rather than vasopressin that promotes pair bonding. It would be very interesting to see a similar study done with women and possible oxytocin receptor polymorphisms. It takes two to tango, after all.
I can just see it now, next month's Cosmo: "How to tell if your man has enough AVP1A, is this relationship over?" "10 Ways to please a man with low AVP1A"...ugh...
H. Walum, L. Westberg, S. Henningsson, J. M. Neiderhiser, D. Reiss, W. Igl, J. M. Ganiban, E. L. Spotts, N. L. Pedersen, E. Eriksson, P. Lichtenstein (2008). Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans Proceedings of the National Academy of Sciences, 105 (37), 14153-14156 DOI: 10.1073/pnas.0803081105

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