(HealthScoutNews) -- Women prefer men who smell like Dad.

That's the translation of a scientific T-shirt-sniffing study that found a woman is attracted to the smell of a man who has immune genes similar to those she inherited from her father.

The research, which appears in the February issue of Nature Genetics, is the first to propose that humans can have such preferences based on a parent's genetic contribution.

But one expert cautions that the study used an artificial experimental design, and that more studies are needed.

The findings grew out of research suggesting that women prefer the scent of men with human leukocyte antigen, or HLA, genes, that differ from their own.

At the time, researchers suggested women use these scent signals to find a mate whose immune genes differ from their own. Theoretically, this would give any children produced by that couple the ability to fight off a greater number of bacteria and viruses than children whose parents have similar immune genes.

HLAs are proteins found on the surface of white blood cells that play an important role in the body's immune response. Everyone inherits a pair of these HLA genes from each parent, yet the millions of combinations of HLA genes vary from person to person.

"This would be a way that genetic information could get out into the environment and be shared," says investigator Martha K. McClintock, a professor of psychology at the University of Chicago.

The latest study looked at scent preferences among 49 unmarried women with an average age of 25 from an isolated German-Austrian community in which the HLA genes are known over two generations. The women had never been pregnant, and were not using any kind of hormonal contraception.

McClintock and her colleagues asked a group of men from diverse ethnic backgrounds to wear cotton T-shirts for two consecutive nights. At the same time, the men were asked to abstain from certain aromatic foods, scented toiletries and sex.

Some of the men had HLA genes similar to those in the isolated community, while others were completely foreign.

The researchers placed sections of the T-shirts in foil-lined cardboard boxes with holes through which the women could smell but not see the shirts.

The women, who were not told the source of the odors, were asked to smell each T-shirt and rate its smell by familiarity, intensity, pleasantness and spiciness. As controls, the women rated sections from unworn T-shirts and those scented with "household smells" of bleach and clove oil. The researchers weren't interested in which smells the women found sexy, only which smells they wouldn't mind being exposed to 24 hours a day.

McClintock found that each woman's most preferred T-shirt came from a donor with significantly more HLA genes that matched her own.

But more importantly, each woman preferred scents from donors who had an average of 1.39 HLA genes matching those she had inherited from her father; her least preferred scents matched only 0.55 genes from her father. There was no significant association with any of the mothers' HLA genes.

While previous studies had pointed to preference for mates who are genetically different, these findings suggest there may be some advantage to a degree of similarity, says McClintock.

And although choosing a mate who is genetically different would avoid inbreeding, she says, "there's a cost to outbreeding."

For example, she says, it would be problematic for a female Chihuahua to mate with a male Great Dane.

"What we've shown is that what seems to be chosen is an intermediate number of [genetic] matches," says McClintock. "Not zero matches, and not 100 percent … because there's dangers at both ends."

Wayne K. Potts, an associate professor of biology at the University of Utah in Salt Lake City, says the findings are compelling but preliminary.

"The idea is that these mating preferences function to produce offspring that are more disease-resistant," says Potts. "The mating preferences themselves could also be functioning to avoid inbreeding, and they could also be used to do other kin-biased behaviors."

However, Potts points out it may be more accurate to study HLA preference based on marriage patterns, rather than by studying brief exposure to boxes containing unknown scents.

And he points out that only 19 percent of the women described the smells as being related to humans.

McClintock says these findings could apply beyond mate choice, extending to a broad range of social relationships.

"I think it has implications for interactions between friends and acquaintances, and in family structure," she says.

What To Do

You can read about mate choice and parenting from the How Humans Evolved Web book, or look through this Society for Neuroscience brain briefing on smell and the olfactory system.

Curious about humans and pheremones? Check out this article from the JunkScience, which discusses previous research by McClintock.

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