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Thursday, April 8, 2004

NATURAL SELECTIONS

What's love gotta do with it?

Scientists try to get a grip on love through MRI scans


This column is often concerned with the evolution of sexual behavior and sexual anatomy, but instead of attributing everything to sex, for once let's accept a view like that of Bertrand Russell.

"Love," he said, "is something far more than desire for sexual intercourse; it is the principal means of escape from the loneliness which afflicts most men and women throughout the greater part of their lives."

So if love is about something other than getting more sex, what is that thing? And -- as this is Natural Selections -- how did love evolve?

Until recently, the questions were entirely metaphysical. That is, we could argue about them but couldn't really answer them, at least not with any scientific rigor. The whole subject was best left to philosophers like Russell. But -- it will be no surprise to learn -- scientists are closing in on love. Before long there should be a framework in place which will help us explain the nature of love and understand its evolution.

Let's get some definitions out of the way first. "What is love?" is a question that could easily fill a column in itself, so let's keep it simple and agree that love is the feeling, bond and deep affection felt between parent and child, and between partners. That is, there is parental love, and there is romantic love. (There is also spiritual or religious love, but we'll get to that later.)

The next question is, how did love evolve? It's quite easy to see why a mother who loves her child will be favored by natural selection. A deep degree of caring, nurturing and protecting -- in short, loving -- clearly helps the survival of a child. A mother might (and sometimes does, in animals as well as humans) sacrifice her life for the sake of her offspring. Such actions by a mother are likely to increase the survival of her offspring (and therefore her genes) and if so they, and love, will be favored by natural selection.

So much for the evolutionary advantage of love. As to the proximate, immediate cause of love, scientists have found that the mother-offspring bond in humans and other animals is mediated by the hormones oxytocin and vasopressin.

What researchers at University College London have now found is that romantic and maternal love activate many of the same regions of the brain. The implication is that maternal love is the evolutionary basis, the foundation, for romantic love.

The researchers, Andreas Bartels and Semir Zeki, of UCL's Laboratory for Neurobiology, also found that love leads to a suppression of neural activity associated with critical social assessment of other people and negative emotions: The brain is told to go easy on people. The findings suggest that once you fall in love, the need to critically assess the character and personality of that person is reduced. The work could provide a neurological explanation for why love makes us blind.

As with so much of our recent understanding of what happens in our brains, the new findings have been revealed by magnetic resonance imaging. In this kind of brain scanning, the subject is placed into a strong magnetic field. A scientist can then locate the precise areas of the brain that are operating at any time by measuring changes in the blood flow -- and thus oxygenation -- of the subject.

Bartels and Zeki scanned the brains of 20 young mothers while the women viewed pictures of their own children, children they were acquainted with, and adult friends (to control for feelings of familiarity and friendship). The work was published in February's issue of NeuroImage. In a previous study (reported in this column), the authors identified the parts of the brain involved in romantic love.

The researchers found that the brain activity recorded in the two studies was remarkable similar. When young mothers gazed at their children, the same brain areas lit up as when young lovers stared at photos of their partners. Both types of love activate specific regions in the reward system, and both reduce activity in the systems necessary for making negative judgments.

"Both romantic and maternal love are highly rewarding experiences that are linked to the perpetuation of the species, and consequently have a closely linked biological function of crucial evolutionary importance," said Bartels. Yet almost nothing is known about their neural correlates in the human.

"Our research enables us to conclude that human attachment employs a push-pull mechanism that overcomes social distance by deactivating networks used for critical social assessment and negative emotions, while it bonds individuals through the involvement of the reward circuitry (regions in the brain that induce euphoric feelings), explaining the power of love to motivate and exhilarate," he added.

And what of religious love? Millions of people could testify to the feeling of rapturous, supernatural love. Is this something else that has flourished on the foundations of the brain laid by maternal love? It's another question that was once thought to be outside the realm of scientific explanation, but with the advances being made with MRI, who can tell?

A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," is published by Shinchosha. Rowan Hooper is a researcher at Trinity College Dublin. He welcomes readers' comments at rowan.hooper@tcd.ie


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