Harvey Karp, the best-selling author of The Happiest Baby on the Block, has some advice on his website for frazzled new parents: “Remember—your baby’s brain was so big that you had to ‘evict’ her after nine months, even though she was still smushy, mushy, and very immature.” It’s not an idea unique to Karp. Scientists have long struggled to explain the myriad challenges attending human childbirth compared to other primates, from the relative helplessness of human infants, to the very “tight fit,” as some researchers have put it, between the female human pelvis and the typical size of a child that must pass through it. More From Our Partners For Male Circumcisions Gone Wrong, a Pioneering Transplant Surgery As Temperatures Rise, the Risks for Pregnant Women May Rise Too Facing Facts: Artificial Intelligence and the Resurgence of Physiognomy The mystery was the catalyst for what became known as “the obstetrical dilemma,” a long-debated though widely accepted hypothesis suggesting that the upright gait of Homo sapiens was accompanied by a narrowing of the pelvis—an evolutionary trade-off that resulted in increased risks to pregnant mothers as they struggled to push large-brained babies through ever-slimmer birth canals. Among other things, the dilemma has been used to suggest that the wider, birth-giving hips of women have hindered them locomotively and athletically—and perhaps even evolutionarily—compared to men. That has always struck some scientists as too pat an explanation, though it is only in the last decade or so that the theory, which still has many subscribers, has received substantive pushback. Today, challenges abound for the idiosyncrasies of human gestation and birth—including new notions that look beyond evolution to more proximate and modern factors like poor diet and obesity. Of course, rigorous debate over the relative strengths and weaknesses of theories in this cul-de-sac of physiological science will surely continue. But for all the back-and-forth, one thing seems quite clear: The days of simply describing the human birth process—and women themselves—as evolutionarily compromised seem to be coming to an end. For some researchers, that