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merits of being able to keep your head while all those around you are losingtheirs. If you can do this, according to his famous poem, “then you will be a man, my son.” Maybe by favoring those women able to “just say no” to the tyranny of estrus, natural selection endowed our female ancestors with the ability to reap the benefits of being more discriminating than any animal (and probably more so than most men, to boot). “If you can keep your secret,” evolution might have been betting, “while all those around you are divulging theirs, then you will be a reproductively successful woman, my daughter.”
    Concealed ovulation could also have adaptively paved the way for women to enjoy greater mating control by reducing the intensity of male–male competition. Think of it this way: If women were only sexually receptive for a day or so each month, at which time they broadcast their availability by sudden, seductive signals of sight, smell, and sexual interest, men might respond by huffing and puffing and blowing each other away—even more than they do now—leaving women little choice, perhaps, but to accept the victor. Dominant males may be desirable sperm donors, if only because their sons may also turn out to be dominant and/or desirable males: This is the so-called sexy son hypothesis (about which more later). But such males may also be terrible fathers, more interested in beating other males over the head than in caring for their own children. By keeping their ovulation secret, and thereby dampening the competitive ardor of men, women might have given themselves the opportunity of choosing men who may be less pushy but more paternal.
    At this point it is worth noting that even though women neither broadcast dramatic ovulatory messages like chimpanzees nor behave in a manner commensurate with estrus in other animals, research has shown that women’s behavior does indeed undergo subtle but substantial changes during their menstrual cycle. When they are ovulating, women are especially likely to wear clothing that is comparatively sexy and reveals more skin, to speak with greater fluency and creativity, to prefer images of men who are especially “manly” as well as favoring male voices that are lower pitched, to be perceived by both men and women as more facially attractive, to have a heightened sense of smell, and literally to move around more. 5
    Moreover, evidence is also accumulating that men are able to perceive at least some of these changes, albeit unconsciously. Thus, in aseries of renowned “dirty T-shirt” experiments, researchers got female college students to sleep alone wearing the same T-shirts for several days during their fertile phase, and again wearing different shirts when postovulatory. Young men were then presented with these pairs of shirts, each worn by the same woman but at different reproductive phases. Sure enough, they preferred the ones worn during the follicular phase (just prior to ovulation) in 75% of the cases. 6 In addition, when the women had been taking hormonal contraceptives, which suppress ovulation, this preference disappeared. 7
    Most dramatic are some findings involving lap dancers. A number of these hard-working women were asked to keep track of two things: their ovulatory cycles and how much money they made on tips while dancing on any given night. The results were staggering, even for people who might have anticipated an effect. During a 5-hour shift while maximally fertile, the women averaged $355; when in the luteal phase (so-so fertility), they brought in $260; and while menstruating, they made a paltry $185. 8 Perhaps these women were sending pheromonal signals to which their male customers were unconsciously responding, or perhaps they unintentionally behaved more sexily as a function of their fertility; either way, they were doing something different and men were responding.
    Evidence of this sort has led biologist Randy Thornhill and psychologist Steven