FULLERTON, CALIF.—If you want to wait by the phone for your next college-aged daughter's call home, you should mark the days of her menstrual cycle on your calendar.

Well, not exactly. But that was one reasonable conclusion of research presented here last week at the 21st annual conference of the Human Behavior and Evolution Society (HBES) at California State University, Fullerton, by Elizabeth Pillsworth, a graduate student in Martie Haselton's lab at the University of California, Los Angeles.

Haselton (pictured at left with David Buss of the University of Texas at Austin) studies sexual attraction, relationships, and how fertility cycles influence mate preferences and choices (for instance, women dress in a more sexually provocative manner during the high fertility phase of the month). In an interesting twist on this body of research, Pillsworth studied the effects of the fertility phase in women on the incest taboo—specifically, how often college-aged women phoned their dads (versus their moms) during the month. Wow. It never ceases to amaze me how clever scientists can be in thinking up new research paradigms: Who would have ever thought of correlating cell phone calls with estrus cycles? Pillsworth and Haselton (and their colleague Debra Lieberman) did! And the results were most revealing.

But first, some background. On the 200th anniversary of Darwin's birth and the 150th anniversary of the publication of The Origin of Species, Darwinian principles have finally come online in mainstream psychology. HBES is the official organization of evolutionary psychologists and a champion of applying Darwinian thinking to human psychology, and its conferences seem to be gaining steam. The last HBES meeting I attended was at the University of California, Santa Barbara, in 1995, and it was sparsely attended. This year, 450-plus attendees packed the tiny conference rooms.

Now to Pillsworth's research: Kin affiliation in evolution is critical for predator avoidance, food procurement and sharing, protection from the elements, and so on. That is, being in a family group is extremely important for mammals. But there is an equally important downside: inbreeding. If you mate with people who are genetically similar to you, there are consequences: higher rates of infant mortality, deformed sperm, sterility, and genetic defects of all sorts—think hemophilia in the royal families of Europe in the 19th and early 20th centuries. Research shows that the offspring of first cousins are twice as likely to suffer congenital malformation and genetic disease and face up to a 5 percent increase in mortality; the offspring of siblings show a 45 percent increased risk of mortality.

Thus, mammals evolved numerous adaptations for inbreeding avoidance: dispersal from natal groups (usually sex-biased), kin recognition and avoidance, and a preference for extra-pair or extra-group copulations rather than copulating within one's group. Pillsworth cited a study on horses that found that mares only leave their group temporarily to join other breeding groups. So there is a conflict of wanting to be close to your kin and kind, but not too close.

The researchers' hypothesis on incest avoidance was that near ovulation, women are motivated to avoid affiliation with male kin (fathers) but not mothers, to avoid the potential costs of inbreeding. Their predictions were that relative to low-fertility days, on high-fertility days women would initiate fewer calls and engage in shorter conversations with fathers, compared to mothers.

They had 51 normally-ovulating women (mean age 19.1 years old) provide complete cell phone bills from one month, along with their menstrual cycle information and details about individuals on their phone bill. It turned out that the subjects called their fathers significantly less than their mothers during high fertility days, and when both mothers and fathers called them during high fertility days they spent less time on the phone with their dads than with their moms.

Conclusion: "this is the first evidence of adaptation in human females to avoid affiliation with male kin when fertility is at its highest."
This study was of particular interest to me because I have a 17-year-old daughter who will be going off to college in a little over a year. Like most parents, I am dreading the day she is gone, when I'll lose my daily contact with her, and am hoping that she calls regularly. I guess I will have to make a mental note of her high-fertility days and expect fewer calls from her—but being that I'm her dad, that won't stop me from calling her and stalling on the phone just to mess with her evolved psychology! Of course, an alternative explanation is that we dads tend to be a bit controlling on matters of our daughters and the boys that love them, and so perhaps daughters avoid talking to their dads during those high-fertility days when they are more inclined to, well, you know ... do things that we dads don't want them to. So instead of “incest avoidance,” an alternative hypothesis might be “controlling dad avoidance.”

Photo of evolutionary psychologists David Buss (left) and Martie Haselton (right) at the HBES conference: Michael Shermer