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Secret of long-term bonding

New Delhi, Dec. 1: The same reward zones in the brain that contribute to human addiction to alcohol or cigarettes also appear to play a role in long-lasting romantic bonds between men and women, new research has suggested.

Scientists in Germany have announced that they have identified the brain mechanisms through which a human male remains loyal to a long-term female partner, practising sexual monogamy, a rare phenomenon among mammalian species.

Neurobiologist Rene Hurlemann at the University of Bonn and his colleagues have found that a chemical called oxytocin may contribute to romantic bonds by making men view their female partner as more attractive and rewarding compared with other women.

“Oxytocin recruits the male brain’s reward system to keep him attached to his female partner,” Hurlemann told The Telegraph. “This action of oxytocin in the reward system could explain why men are motivated to stay in a monogamous relationship.”

The researchers gave intranasal sprays of oxytocin to men who had female partners and used magnetic resonance scan imaging to study their brain activity while they watched photographs of the faces of their partners, familiar women or unfamiliar women.

They found that oxytocin stimulated the reward system in regions of the brain called the ventral tegmental area and the nucleus accumbens in men in response to viewing the faces of their partners, but not as much when they observed the faces of other women. The findings were published last week in the US journal Proceedings of the National Academy of Sciences.

The link between oxytocin and monogamy first emerged in the 1970s when biologists discovered that in rodents called prairie voles — which are monogamous —the nucleus accumbens was brimming with receptors, or gateways, for oxytocin.

Studies have also shown that tampering with the oxytocin receptors changes the monogamous behaviour of prairie voles. Such observations have led scientists to suggest that pair bonding is social behaviour linked to oxytocin receptors in the nucleus accumbens.

In a study published last year, Hurlemann and his team had shown that oxytocin-treated men who were already committed to long-term romantic relationships kept a social distance from an attractive female, while single men did not.

Those results suggested that oxytocin helps maintain ongoing relationships. The new findings have revealed the underlying mechanisms — oxytocin-driven activity in the ventral tegmental area and the nucleus accumbens — centred in the reward zones of the brain.

“The action of oxytocin in the reward system could explain why men are motivated to stay in monogamous relationships,” Hurlemann said. But activating the reward mechanism requires high levels of oxytocin in the brain. The high levels, Hurlemann said, may be maintained through acts of hugging or touching, typical behaviour exhibited by couples. Natural oxytocin is produced in a region of the brain called hypothalamus and other sites in the body.

The findings that oxytocin recruits the reward system to promote monogamy is similar to how certain drugs create some form of dependence or addiction. “Many people crave for partners after a broken relationship,” Hurlemann said.