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The finding adds to the growing body of evidence suggesting that our natural smell influences other people on an unconscious level, and strengthens the argument that human pheromones exist and still exert a subtle influence over us.

In the study, smells associated with breastfeeding increased feelings of sexual intimacy in childless women volunteers. Why this should be so is a mystery, but the researchers suggest it may be a way that women signal to each other that the environment is a good one in which to reproduce.

Julie Mennella of the Monell Chemical Senses Center in Philadelphia and a team at the University of Chicago asked 26 nursing mothers to wear absorbent pads in their bras and under their armpits. The odors collected on the pads probably came from both the mother and the feeding baby.

Another 45 women, who had never given birth, then spent the next three months undertakng a "sniff challenge". For a month, all the women sniffed control pads with a phosphate buffer on them four times a day. For the final two months, some women were randomly chosen to sniff pads with the breastfeeding compounds, while others continued with the control scent. Each day the volunteers measured their temperature, took a urine sample and recorded sexual activity.

Last year, Mennella's group showed that exposure to breastfeeding odors disrupted the menstrual cycles of volunteers: longer cycles got longer and shorter ones got shorter.

The new study reveals a more subtle effect. While the women smelling the breastfeeding compounds did not report increased sexual activity -- this behaviour was most obviously influenced by the absence or presence of a partner -- they did report significantly heightened and more enduring sexual desire and fantasies. "The data are pretty striking," says Mennella, who presents her evidence this week to a meeting of the Association for Chemoreception Sciences in Sarasota, Florida.

She concludes that the chemicals encourage other women to reproduce, and that they may have evolved as a signal that the environment is suitable for raising young. In many cultures, newly-wed young women are encouraged to spend time around new mothers to increase their own chances of having children, she says. "I wonder if these cultures have tapped into something." She is eager to find out if the breastfeeding smell has any impact on fertility.

Richard Brown, a psychologist at Dalhousie University in Halifax, Nova Scotia, notes that these are only preliminary findings. But he points out that breastfeeding women have higher than normal progesterone levels. "Maybe the high progesterone acts like an androgen," he speculates. "Maybe it's the weirdest of possible things and they're producing male-like odors."