Men can tell by smell alone when women are at their most fertile, say researchers from Texas.

They reached this conclusion after getting men to sniff T-shirts worn by women during fertile and infertile stages of their menstrual cycles.

Overwhelmingly, men rated the smells from the T-shirts to be most "pleasant" and "sexy" when they had been worn by women during their fertile phase.

"It may be a cue that men pick up subtly," says Devendra Singh, the psychologist at the University of Texas in Austin who made the discovery. But Singh doesn't think the smell would play as important a role as visual cues in determining sexual attractiveness.

The findings shed light on how our ancestors selected mates and may even open new avenues for treating infertility.

Fresh shirt

Singh and his colleague, Matthew Bronstad, gave a pair of previously unworn T-shirts to women volunteers.

Women slept in the first T-shirt during the fertile phase of their cycle - 13 to 15 days after their previous period. They slept in the other T-shirt during the infertile phase on days 21 to 22 of the cycle.

The women avoided contact with strong odours to avoid masking subtle scent changes. They washed with unscented toiletries, avoided sex and gave up spicy or aromatic foods, such as garlic. Two women had to drop out after indulging in pizzas and cigarettes.

When men sniffed the 21 pairs of T-shirts, 17 gave a resoundingly higher rating to the smells from the fertile phase. They had been told that one set of T-shirts came from women who were more attractive.

The findings were the same when a second set of men sniffed the T-shirts after they'd been stored for seven days. "We wanted to see how stable the smell was," says Singh.

Advertising strategy

He says that the odours probably enable women to advertise their readiness for fertilisation. But in the modern world they are probably overwhelmed by more obvious cues such as dress, make-up and behaviour.

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More at: Proceedings of the Royal Society, London B (vol 268, p 797)

1343 GMT, 4 April 2001