There may very well be logical explanations for these apparent empirical shortfalls. For example, modern food regulations and prenatal care may counteract the effects of a deleterious diet today, but there may indeed have been more frequent birth defects in ancestral infants whose mothers weren’t susceptible to pregnancy sickness. And perhaps other species do exhibit these symptoms, but they are simply more difficult to detect in the wild than in the more familiar lab monkeys or dogs. Yet Gallup suspects we’ve been barking up the wrong tree with the Hook-Profet hypothesis. “Diet may only be a small part of the picture,” he wrote to me by email.
So what does Gallup say is the real culprit behind nausea and vomiting in early pregnancy? Semen. More specifically, unfamiliar semen. To understand where he’s coming from, we need to think back to the maternal immune system’s response to the fetus. Because half of the DNA the fetus is carrying comes from the father, the mother’s body may initially treat the organism as foreign tissue or an infection. This response, Gallup says, triggers an immune reaction that is commonly experienced as nausea, vomiting, and malaise (aka morning sickness). The best cure for this type of sickness, says Gallup, is, strangely enough, the same thing as its cause. The more exposure a woman has to her partner’s semen—that is to say, the more often she’s inseminated prior to conception and during the early stages of the pregnancy—the more tolerance her body develops to his genetic material. This tolerance generalizes to a tolerance for the fetus and leads to successful maternal immunosuppression—and subsequently allows her to feel less like an infected zombie with serious stomach troubles.
Here is where Gallup’s reputation as an innovative—if often highly speculative—evolutionary theorist comes into the picture. Gallup surmises that pregnancy sickness is not itself an adaptation, but instead a side effect of a broader maternal adaptation for favoring the best possible mates. He suggests that this broader adaptation serves primarily to facilitate reproduction with males that are likely to support mother and child (in evolutionary terms, to invest in the offspring), while weeding out the players. In previous work, Gallup has shown that women are more likely to develop preeclampsia—and thus have a higher infant mortality risk—in pregnancies resulting from unfamiliar semen. Historically, these would have included rape and “dishonest mating strategies” (tactics in which the man lies to the woman about his long-term intentions just to get into her pants) as well as unplanned conception occurring in a new, still-fragile relationship. From the point of view of Mother Nature’s cold, cold heart, spontaneous abortions due to a reaction against unfamiliar semen might have been biologically adaptive. This is because conception and childbirth historically meant that a woman foreclosed on any other reproductive opportunities for 2 to 4 years, so pregnancies in which paternal investment was improbable would have meant an enormous gamble. Today, however, technological innovations such as barrier contraceptives (condoms reduce a woman’s exposure to semen that would otherwise become familiar) and artificial insemination mimic some ancestral conditions. The maternal immune system has no way to distinguish between, say, conception by in vitro fertilization and rape.
Gallup’s evolutionary reinterpretation of pregnancy sickness is quite new—so new, in fact, that it hasn’t been put to a test. But at the 2012 meeting of the Northeastern Evolutionary Psychology Society in Plymouth, N.H., he and graduate student Jeremy Atkinson laid out a set of explicit predictions that, if borne out by data, would support their model and may lead scholarship away from the traditional embryo-protection account. First, the authors predict that the intensity of pregnancy sickness should be directly proportional to the frequency of insemination by the child’s father. “Risk factors for morning sickness,” they reason, “should include condom use, infrequent insemination, and not being in a committed relationship.” In fact, Gallup and Atkinson believe that lesbians with little (if any) previous exposure to semen who are impregnated by artificial insemination should have some of the worst cases of nausea and vomiting. Also, pregnancy sickness should wane in severity from one consecutive pregnancy to the next, but only assuming that the same man sires each successive offspring. By contrast, a change in paternity between offspring should reinstate pregnancy sickness.
Although the Hook-Profet view has long been considered to be the solution to the nausea gravidarum mystery, the full evolutionary story may have yet to be told. Still, while it may very well work wonders in curtailing nausea and vomiting in your next pregnancy, it’s probably too early to suggest imbibing copious amounts of your partner’s semen, either vaginally or orally (there is some evidence, believe it or not, that fellatio may be just as effective as vaginal insemination for priming the woman’s body with the man’s protein, activating maternal immunosuppression in preparation for a child.) Your partner might think it’s worth a shot, though.