Friday, June 11, 2010

Oh what a lovely molecule!

The continuing adventures with the astonishing hormone oxytocin

Oxytocin is small but remarkable molecule. It clocks in at just nine amino acids — compared to 524 for our blood’s oxygen carrier, haemoglobin — yet it packs a powerful punch. And where haemoglobin is tasked only with the relatively simple job of ferrying oxygen around the blood, the effects of oxytocin reach into some the deepest recesses of the human condition.

Oxytocin acts as both a hormone that circulates around the body and a neurotransmitter that regulates brain activity. Over the past 10 years, oxytocin has been implicated in some of the most fundamental aspects of social relationships, such as romantic love, trust and bonding: it helps create the strong bond between infants and mothers, reduces maternal stress, increases trust in economic games, and ameliorates anxiety.

Many studies have stressed the importance of tactile contact in regulating the effects of oxytocin. Touch is an important means of social communication for many mammals, and is frequently deployed to convey emotional states, such as friendliness and anxiety. And studies on rodents have suggested that direct tactile contact is indeed essential for activating the oxytocin system.
Yet along with physical contact, vocalisations — such as the squeaks of mice — are also an important means of social communication in a range of mammals. And oxytocin plays a key role here, too. Mice genetically engineered to lack the oxytocin gene produce fewer social vocalisations, show profound social deficits, and have higher levels of the stress hormone cortisol.

This led Leslie Seltzer, Toni Ziegler and Seth Pollak from the University of Wisconsin-Madison to wonder whether such vocalisations could boost oxytocin levels. And while rats might be quite vocal, humans have taken this skill to an extreme. So the team looked at whether human speech alone could affect oxytocin levels.

Comforting words
Seltzer and colleagues enlisted more than 60 mother-daughter pairs for their study. All daughters were prepubescent but old enough to understand the experiment, and the experiment was limited to females to ensure the greatest degree of similarity and comparability between subjects, and also to connect their results to earlier work, most of which has focused on females. To test the hypothesis that the spoken word could cause the release of oxytocin, each poor kid had to speak and solve maths puzzles in front of an audience, a task well known to induce stress and the release of cortisol.

Then the kids were split up into three groups. One group was then reunited with their mothers for 15 minutes, who comforted them with hugs and soothing words; this comprised the ‘complete contact’ condition. Another group received a telephone call of the same length from their mothers (‘talk only’); the final third (the control group) watched an emotionally neutral film for 75 minutes (the other girls watched 60 minutes of the film after seeing or speaking with their moms, so that the effects of the film could be subtracted out of the analysis).

Their results — reported a few weeks back in Proceedings of the Royal Society B — support British Telecom’s slogan from the 1990s that “It’s good to talk”. In all the stressed-out kids, cortisol levels spiked, as to be expected. Yet in both the complete-contact and talk-only groups, cortisol levels returned to normal more rapidly than in controls. This was accompanied, in the experimental groups, with elevated urinary levels of oxytocin, whereas controls showed no rise in the hormone. And although the magnitude of these effects was greater with complete contact compared with a call alone, cortisol levels in both groups were statistically indistinguishable after an hour. In the absence of a comforting hug, a few words of support may do the trick in soothing a stressed-out soul.

To tend and defend
Another study on oxytocin, published in today’s issue of Science, explores its effects in males — and not as an anti-stress hormone but as the source of social solidarity. Humans are the social species par excellence. Yes, ants and bees and many other species in huge groups, but none cooperates with genetically unrelated individuals to the same extent as humans. This capacity for large-scale cooperation, and the altruism it is built on, is the key to the global success of humans over the past 50,000 years.

Yet altruism isn’t cast about indiscriminately. It is directed more towards members of variously defined in-groups (only a small percentage of whom will be relatives), and selectively withdrawn from perceived out-groups. This ‘parochial altruism’ may have emerged through the process of cultural group selection, in which groups fostering prosocial norms towards the rest of our group, and antagonism towards out-groups, led to greater success in inter-group competition.

Such cultural selection can also have effects on biology — a phenomenon biologists call gene–culture co-evolution. As famous example is dairy farming: among those populations that picked up the cultural habit of keeping cattle and using their milk, genes for digesting lactose beyond childhood became more valuable and spread, so we now see high levels of lactose tolerance in societies with a history of dairy farming, and low levels elsewhere. In the case of altruism, in-group amity and out-group enmity, a cultural milieu favouring in-group love could have placed a premium on biological mechanisms that promote this feeling.

So Cartsen De Dreu and colleagues decided to see whether oxytocin played any role in modulating parochial altruism. Their experiments, like so may that investigate altruism and inter-group cooperative dynamics, used simple economic games — and in this case, only males were included. The games they played went as follows. Each player was assigned to a three-person group (their in-group), which was paired up with another three-person group (the out-group). Each player was given 10 Euros, which they could either keep to themselves, or contribute to one of two common pools — a ‘within group’ pool and a ‘between group’ pool — after which the pooled money was to be split among members of each group.

These allocation options came with different economic payoffs for the parties involved. For every Euro contributed to the within-group pool, an additional 50 cents were given to each member of the player’s in-group, and so contributions to this pool measured ‘in-group love’ (a purely selfish player would keep all their money to themselves while hoping to free-ride on the contributions of their group members, of which they would get a share when the post was divvied up). Paying into the between-group pool also generated an extra 50 cents for each fellow in-group member, while also decreasing the money in the out-group by 50 cents per player, and so provided a measure of out-group hate.

Before playing this game, some participants received a shot of oxytocin, administered as a nasal spray. (This method has been used in previous studies, where it was shown to increase feelings of trust.) This hormonal boost had the effect of increasing in-group love, but had no consequences for out-group hate. 52% of players in the placebo condition behaved selfishly and did not contribute to the within-group pool, and only 20% showed in-group love. The single shot of oxytocin, however, switched this patter, and now only 17% of players acted selfishly, with 58% evincing in-group love. (Out-group haters comprised 28% of players in the placebo condition, and 25% in the oxytocin arm.) This warm glow of in-group love also moved those who scored more highly as selfish on questionnaires to contribute more to the in-group, so it’s the case that only those primed to cooperate respond to oxytocin in this way.

Other experiments suggest that the in-group love is driven less by hatred of the out-group and more by a desires to protect the in-group. Players played similar economic games, but this time the rules were manipulated so that in some conditions players could do particularly well by cooperating with their teammates, which is this case meant contributing a decent share of their endowment (this was the ‘greed’ situation). At the same time, the rules also allowed for the possibility that players would do particularly badly if they failed to work together cooperatively, while the out-group would gain a significant edge (the ‘fear’ condition). So some games were in greed and fear, other high in one and not the other, and some low in both. This enabled the effects of greed and fear on cooperation, and this interacted with oxytocin.

As expected, those given oxytocin showed more in-group love than those receiving placebo. But this effect was greatest when players were in a high fear situation — that is, when the possibility that their group would lose out heavily was salient. Meanwhile, no effect of greed was seen.

It needn’t have been this way. Parochial altruism could promote in-group love and, simultaneously, fuel aggression towards out-groups. But in these experiments at least, parochial altruism emerged as a ‘tend and defend’ philosophy: look after your own and protect them, but don’t go all out to get at out-groups. And as social philosophies go, it’s not the worst starting point in the world.

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