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boost performance on tests of odor memory and odor identification. Another factor is hormones. A woman’s smell sensitivity varies across her menstrual cycle and is greatest around the time of ovulation. Hormone effects are not simple; they interact in complex ways with cognitive factors. This interaction produces one of the most dramatic olfactory sex differences ever observed in the lab. Sensory researchers Pam Dalton and Paul Breslin tested men and women for their sensitivity to a specific odor. With repeated testing over the course of thirty days, the women became much more sensitive to the odor, while men did not. The effect was confined to the tested smell; sensitivity to a different odor did not change for men or women. The enhanced sensitivity can’t be attributed to practice; the women weren’t getting better at threshold tests in general. They became more sensitive because they paid close attention to low levels of odor while being exposed to it multiple times. Most remarkably, Dalton and Breslin didn’t find enhanced sensitivity in prepubescent girls and postmenopausal women. The phenomenon is limited to women of reproductive age. This implies that female hormones are needed to make it happen, and in fact it can be observed in postmenopausal women who take hormone replacement therapy.

Sex differences are evident within days of birth: baby girls turn toward novel odors and spend more time smelling them than baby boys do. The anthropologist Lionel Tiger attributes the difference to evolution. In our long history as hunter-gatherers, he says, it was the females who gathered fruits and vegetables, and a good sense of smell was valuable in judging ripeness and safety. Tiger’s view—essentially a biologized version of “women spend more time cooking”—will not be received warmly in some quarters. Yet it’s hard to see how a cultural explanation can explain sex differences in two-week-old infants.

WITH AGE, OLFACTORY performance begins to deteriorate. The first signs of decline are detectable in the early forties—at least under laboratory conditions—and the pace accelerates in the sixties and seventies. Interestingly, the rate of decline varies with the odor. Rose and banana, for example, are easily perceived until people are in their seventies, while mercaptans (the natural-gas warning odor) show a drop among people in their fifties. Some age-related smell loss can be traced to the nose itself—the accumulated wear and tear of infections and minor blows to the head. Some of the loss is traceable to the brain. For example, odor identification ability depends on how much short-term memory the test requires. Because short-term memory declines with age, elderly people score better when the odor test is presented in a simple yes/no format than in a multiple choice format that requires more memory capacity. In any case, decline is not inevitable; a given seventy-five-year-old may outperform a given twenty-five-year-old. Perfumers, in fact, usually get better with age. Experience and skill more than compensate for any dimming of acuity that comes with age. I know of no fragrance house with a mandatory retirement age for perfumers.

TO THE AVERAGE person it seems obvious that smoking must dull the sense of smell. Surprisingly, the evidence is equivocal. Some studies find adverse effects of smoking but many, including several recent ones, do not. One, an Australian study of 942 people, found that having a smoke within fifteen minutes of smell testing put a temporary dent in performance. Other than that, “smoking did not reduce olfactory performance or self-assessment of olfactory ability in this group, contrary to previous findings.” The National Geographic Smell Survey reported mixed results. For example, smokers found the artificial musk scent of Galaxolide more intense than did nonsmokers, but the reverse was true for the musky-urinous smell of androstenone. Pleasantness ratings for the skunky-smelling mercaptan sample were higher among smokers, but so were their ratings for rose and cloves. It’s possible that smokers become