Brown University professor Rachel Herz describes her findings on the importance of the sense of smell in her book, The Scent of Desire. She says everyone has an “odorprint” as unique as the fingerprint.

The Providence Journal / Sandor Bodo

Quick, ladies, what is the one thing that most attracted you to that special someone in your life? His eyes? His smile? The dimple on his chin? His intelligence and wit?

Wrong, all wrong.

It was his smell.

Not his cologne or aftershave. His smell, his natural smell.

You may or may not be conscious of the fact, you may not quite believe it, but chances are it was body odor, above all other physical characteristics, that told you this guy is interesting.

Men are similarly attracted or repelled by women’s body odor, but for them (the beasts!) visual cues are more important. Especially a hot body.

Both reactions, as it happens, make evolutionary sense, according to psychologist Rachel Herz of Brown University.

She makes these and other fascinating claims in her new book, The Scent of Desire, to be published this week by William Morrow. Despite the lurid title, the book is redolent of scholarship and steeped in data from scientifically controlled experiments, with only an occasional vagrant whiff of anecdotal evidence or speculation.

“I was surprised, too,” Herz said the other day, acknowledging the usual reaction. “I mean, especially because the actual awareness (of smell) is quite low.” She was interviewed in her East Side office in the Hunter Psychology Laboratory at Brown University, which she retains although she has shifted from the psychology department, which she joined in 2000, to the medical school’s department of psychiatry and human behavior.

But the fact is, she said, time after time, when college students are asked to rate the positive importance of body odor along with several characteristics more likely to be found on an eHarmony.com questionnaire, odor trumps almost everything.

For women, it beats out all other physical characteristics; for men, all but appearance — and for both sexes, body odor comes perilously close to outscoring all non-physical characteristics as well. In her most recent test, with Brown students, the only factor that got higher ratings from both sexes was how pleasant or nice the other person seemed. Women rated personality just above smell, men above appearance and smell.

Odor even beat out IQ. “Intelligence came very, very close to smell for women, so it was pretty much tied, but not exactly,” Herz said.

That data may dismay Brown’s admission officers and confound anyone running a dating service, but it makes eminent sense from an evolutionary point of view, Herz says.

From a strictly Darwinian perspective, all human (and animal) courtship and social behavior develops from and responds to one imperative: Reproduce! So it’s easy to see why men are attracted to women with hourglass figures — visual evidence of a capacity to bear children.

But why is body odor important? And why especially for women? Here the argument gets more complex — and interesting, at least to the scientifically inclined.

It turns out that it’s all related to our immune systems.

“I GOT INTERESTED IN THIS,” Herz said, “because the sociobiological argument about women is that they don’t really care about (men’s) physical features: they are interested in the social features, like status and resources and how many baubles you can bring to the nest, and so on.

“And that struck me as strange, based on evolutionary biology…. From a female perspective, the signals that relate to the health potential of the possible child she may have with a man should be highly, highly important — the most important thing, really. Forget about money; if the kid’s going to die before he’s 11, then it really doesn’t matter.

“So the feature that is the most significant with respect to individuals mating and producing a child is their immune systems. And, it turns out, quite amazingly, that the phenotype for your immune system is your body odor.”

In other words, your particular body odor is the outward manifestation of the genetic makeup of your own particular immune system — which is unique to you, unless you have an identical twin. Thus your “odorprint” is as distinctive as your fingerprint.

IT ALSO TURNS OUT, AS Herz explains in the book, that babies have a better chance of survival into their own reproductive years if their parents’ immune systems are different from each other, providing their offspring with protection against the broadest possible range of diseases.

Evolutionary biology, therefore, would predict that males and females are attracted to body odors distinctly different from their own. And that turns out to be precisely the case.

The first experiments were with mice, whose immune system genetic patterns could be controlled through breeding. The female mice chose males whose patterns were different from their own, and did so based on odor.

Selective breeding of humans is out of the question, but once medical science had progressed to the point where genetic structures could be mapped (it was found that more than 50 genes are involved in the immune system, and that their patterns were the most variable in all of nature), the groundwork was in place for a similar experiment. It was conducted in Switzerland by Claus Wedekind and his colleagues, who reported their findings in 1995.

As Herz described it in her book, the immune systems of a group of young men and women were genetically profiled. The men were then asked to wear T-shirts to bed for two nights. The shirts were collected and placed in identical cardboard boxes. Each woman then sniffed the boxes and was asked to choose the T-shirt she thought had “the sexiest and pleasantest” smell. The women consistently picked ones worn by the men whose immune systems were most unlike their own.

“So the importance of body odor for women from a biological perspective seems very, very large,” Herz said. “The other part of that story is that women’s sensitivity to smell is highest during ovulation, and that would be the time when she should be the most sensitive to determining who would be the right mate or not, because the stakes are high.”

WHY, THEN, IS SENSE OF SMELL the Rodney Dangerfield of the senses, getting no respect?

“Since the Victorian era, the sense of smell has been relegated to the base and sexual and dark and dirty and bad — and actually was quite stigmatized, I think, for a long time,” Herz says. “Trygg Engen,” the retired Brown psychologist whose pioneering work in the field brought her here from Toronto, “was truly the first person to legitimize it within an empirical context.”

The study of olfaction has gotten more respect since 2004, when two researchers in the field won the Nobel Prize. But it remains in bad odor, so to speak, in some scientific precincts because measurement and quantification are nearly as elusive as the breeze that brings a hint of lilacs on a summer day.

“Even in this department the value of it hasn’t necessarily been appreciated to the extent that it could have been,” Herz says.

It is even more despised among the rest of us. In a recent survey Herz cited, respondents were asked to rate the value of losing various physical attributes, and small came in tied with losing a big toe.

Big mistake, Herz says.

THE TASTE OF FOOD is almost entirely a matter of smell, she points out: no smell, no flavor. That has been known for a long time. Less well known, she says, is the direct link between scents and emotions. Cases of depression or other emotional disturbances among people who have lost their sense of smell are well documented. Here too, as in the case of the link to courtship, there appears to be an evolutionary explanation.

Herz was led there after tackling another question, the one that got her interested in olfaction in the first place: Is it true, as most experts in memory believe, that a particular smell can trigger a memory more vividly and completely than any other sensory cue?

It turns out, she says, that memories triggered by smells are no more detailed or reliable than any others.

“But the big difference is the feeling of emotion that comes with the memory, and also the evocativeness, the feeling of being back in time and place in that moment. That sort of visceral component of it is very distinctive and really unique, and it ties in with the limbic system.

“The neuro-evolutionary story is that the olfactory cortex ,” the part of the brain that processes odors, “was the first part of the limbic system that evolved, and the amygdala” — the part involved in emotions — “grew out of it. And if you think about it, the purpose of smell is to tell you to go forward or to go away, and the purpose of emotion is the same thing. So metaphorically, functionally, anatomically, etc., the two are almost versions of one another in different ways. It’s really a very special relationship emotions have to olfaction.”

So, a little more respect, please, Herz urges. And the next time you hear two lovers talking about the “chemistry” between them, you may allow yourself a knowing smile.

driggs@projo.com