Scientists are finding that, after all, love really is down to a chemical addiction¹ between people.

OVER the course of history it has been artists, poets and playwrights²（剧作家） who have made the greatest progress in humanity's understanding of love. Romance has seemed as inexplicable³（费解的） as the beauty of a rainbow. But these days scientists are challenging that notion, and they have rather a lot to say about how and why people love each other.

Is this useful? The scientists think so. For a start, understanding the neurochemical pathways that regulate social attachments⁴ may help to deal with defects in people's ability to form relationships. All relationships, whether they are those of parents with their children, spouses⁵ with their partners, or workers with their colleagues, rely on an ability to create and maintain social ties. Defects can be disabling, and become apparent as disorders⁶ such as autism and schizophrenia（精神分裂症） —and, indeed, as the serious depression that can result from rejection⁷ in love. Research is also shedding light on some of the more extreme forms of sexual behaviour. And, controversially, some utopian（乌托邦的） fringe groups see such work as the doorway⁸ to a future where love is guaranteed because it will be provided chemically, or even genetically⁹ engineered from conception.

The scientific tale of love begins innocently enough, with voles. The prairie vole（野鼠） is a sociable¹⁰ creature, one of the only 3% of mammal species that appear to form monogamous（一夫一妻的） relationships. Mating between prairie voles is a tremendous 24-hour effort. After this, they bond for life. They prefer to spend time with each other, groom¹¹ each other for hours on end and nest together. They avoid meeting other potential mates. The male becomes an aggressive guard of the female. And when their pups are born, they become affectionate（深情的） and attentive¹² parents. However, another vole, a close relative called the montane vole, has no interest in partnership¹³ beyond one-night-stand sex. What is intriguing¹⁴ is that these vast differences in behaviour are the result of a mere¹⁵ handful of genes¹⁶. The two vole species are more than 99% alike, genetically.