The New York Times,
June 9, 1998
WHEN it comes to love, sex and friendship, do birds of a feather flock together? Or is it more important that opposites attract? The argument is so old that even Aristotle mentioned evidence for both sides.
Recently, a new chapter has been opened by researchers who say that for at least one type of gene people find difference sexy and sameness boring -- and that they use their noses to tell which is which.
Among the recent studies are those of a Swiss group that used sweaty T-shirts to establish that people can sniff out genetic difference, a Chicago team that concluded from its study of a religious community that genetically similar people tend to avoid marrying one another, and a New Mexico study, again using T-shirts, that claims women at their most fertile time of month will prefer the odor of the fittest-looking men.
Some other scientists regard these claims as spurious, but more research in this vein is in the works, the believers say.
The genes in question instruct cells to make the proteins of the Major Histocompatability Complex, one of the immune system's key markers of identity. MHC proteins attach to foreign bodies and present them to the immune system for a verdict of self or not self. The system attacks anything that does not pass the test. That includes foreign MHC proteins, which is why skin grafts and transplanted organs are rejected unless the donor's MHC is very similar to the recipient's.
There are more than 100 MHC genes on human Chromosome Six and so many versions of each gene, that in a typical population of 100,000 people, only two or three people are likely to match very closely.
''If you think about how hard it is to find organ donors, that's what we're talking about,'' said Carole Ober of the University of Chicago's Department of Human Genetics, who is running the Chicago studies. ''It's virtually impossible to find a match if you go outside your own family.''
That unusual variety makes the MHC genes useful to population geneticists, who track ancestries by tracing combinations of versions of genes, which are passed to offspring in sets called haplotypes.
In 1974, the writer and medical researcher Lewis Thomas suggested that different MHC genes might be linked to different odors. He was considering training dogs to sniff out compatible people for skin grafts and organ donations.
That led to work on laboratory mice and rats that established that the animals preferred mates that were different in their MHC genes. Scientists speculated that this was either a mechanism to prevent inbreeding or a way for animals to insure that their offspring would have immune systems diverse enough to fight as many diseases as possible.
In the last few years, several researchers began to wonder whether the MHC effect in rodents might be found in people.
Claus Wedekind, of the Zoological Institute at Bern University in Switzerland, believed that body odor might signal that its owner had desirable immune genes that would help offspring fight off diseases. He devised an experiment to see if human body odor correlated with MHC genes and if people could tell.
He and his team collected DNA samples from 49 female students from the university, mostly in biology and psychology, and 44 male students, mostly from chemistry, physics and geography. He asked the men to wear cotton T-shirts on a Sunday and a Monday night, to keep the shirt in a plastic bag, to use perfume-free detergents and soaps and to avoid smelly rooms, smell-producing foods and activities, like smoking and sex, that create odors. Meanwhile, the women were given a nasal spray to use for two weeks before the test to protect their nasal membranes from infection. They also each got a copy of the Patrick Suskind novel ''Perfume'' to make them more conscious of odors.
After the T-shirts were collected, each woman was asked to give ratings, for intensity, pleasantness, and sexiness, to three T-shirts from men with similar MHC genes and three from men whose MHC genes were less similar. They did not know which shirts were which.
''Women who are not taking oral contraceptives and who are dissimilar to a particular male's MHC perceive his odor as more pleasant than women whose MHC is more similar to that of the test man,'' Dr. Wedekind and his colleagues wrote in the Proceedings of the Royal Society of London, where the results were published in 1995.
Odors of men with dissimilar MHC reminded the women of their own mates or former mates twice as often as did the odors of men with similar MHC. However, if a woman was taking oral contraceptives, which partly mimic pregnancy, this predilection was reversed, and they gave higher ratings to men with similar MHC. ''The Pill effect really surprised me,'' Dr. Wedekind said in an interview.
In a follow-up study, Dr. Wedekind was surprised to find that no particular combination of MHC genes seemed more desirable than any other. Rather than being impelled to seek ''good'' combinations of MHC genes that were unusually resistant to disease strains, as he had expected, he found that his subjects seemed simply to find difference appealing.
On a different research track, Dr. Ober and her Chicago colleagues studied 411 married couples in 31 Hutterite colonies in South Dakota.
A tight-knit religious group who marry among their own and believe in large families, the Hutterites in her sample are unusually closely related to one another because they are all descended from only 64 people who emigrated from the sect's original home in Europe in the 1870's. That means couples do not have American society's astronomical odds against sharing MHC genes.
Dr. Ober's team sampled the DNA of all 411 couples, identifying haplotypes
on the MHC region of Chromosome Six. She then entered the known genealogy of
the Hutterites into a computer simulation, which predicted significantly more
haplotype-matched couples than actually exist. That suggests, Dr. Ober said,
that Hutterites somehow avoid marriage to partners with whom they share too
many MHC genes.
Dr. Steven Gangestad, a psychologist, and Dr. Randy Thornhill, a biologist, both at the University of New Mexico, have proposed that symmetry between right side and left, which in many animals is a sign of health, fertility and longevity, is also important in human mating.
Dr. Gangestad and Dr. Thornhill measured their male volunteers for ear length, ear width, elbow width, wrist width, foot breadth and lengths of fingers and thumbs. In a paper to be published this year in the Proceedings of the Royal Society of London, they wrote that female volunteers who were not taking oral contraceptives preferred the smell of T-shirts worn by the most symmetrical men only if the women were at that stage of their monthly cycle in which they were more likely to become pregnant. In fact, the doctors wrote, the preference was stronger as the likelihood of pregnancy increased.
Many other biologists are not convinced by these studies. ''I'm basically skeptical until I see some really strong evidence,'' said Philip W. Hedrick, a biologist at Arizona State University, and a critic of the idea that smell affects human sex and marriage. ''I think mate choice is probably a lot more complicated, particularly in humans.''
Last year, in the issue of The American Journal of Human Genetics that published Dr. Ober's paper on Hutterite mate choice, Dr. Hedrick and Francis L. Black of Yale University reported that their analysis of couples in 11 Amazon tribes had not found the same effect. They looked at two MHC genes, while Dr. Ober used haplotypes, so the studies are not strictly comparable.
Dr. Hedrick has also clashed with Dr. Wedekind. The Swiss team ''basically trained his subjects to make odor significant,'' with their nasal sprays and novels, Dr. Hedrick said.
The scientific debate will continue as biologists plan further studies and the technology for precisely measuring odor improves, Dr. Wedekind said. In the meantime, he and fellow believers must also contend with part of the lay public that is uncomfortable with research that suggests that profound decisions about sex, marriage and parenthood could be affected by body odor.
''One politician called and told us she would be doing everything she could to see I was expelled from the university,'' Dr. Wedekind said. ''Some people really fight this kind of research.''
Dr. Wedekind said he was unsympathetic to those fears. The key words used to describe the research for reference, he said, include humans, mate choice and genes. ''That makes some people want to fight it, and I understand their concerns,'' he said. ''Some people may be remembering what happened 60 years ago in Germany.''