There is a curious disconnect when it comes to studying sex differences. In the Humanities it has become fashionable to minimize them, because we seem to have become afraid that if we look too deeply into them we may find out that there are cognitive differences between men and women, and from there to say that one sex is more intelligent than the other there is just a small step. That fear is probably not justified, as Stephen Jay Gould explained in “The Mismeasure of Man”. Meanwhile, in the Sciences there has been a big push to study sex differences. This was done for a feminist cause: it became apparent that most research was done on male animals and therefore its findings did not always apply to women's diseases. The turnabout was so drastic that today you cannot write an NIH grant proposal without explaining how you are going to study sex differences or giving a justification for why you are not going to do that. And, for sure, once you include sex as a variable, differences pop up all over the place. For example, men and women have different sensitivities to pain, and their pain responses change differently in the stages of life or under conditions like pregnancy and childbirth. Testosterone makes men more aggressive and risk-prone than women. Bonding is mediated by oxytocin in women and vasopressin and oxytocin in men. And, when it comes to cognition, on average men do better at spatial tasks and women at verbal tasks. The sex hormones, testosterone, estrogen and progesterone, have receptors in most neurons that affect the way they function. When men lose testosterone as they age they tend to become depressed and less motivated.
Here are a few citations on sex differences. There are mostly on pain perception because that is my field of research. This is just a tiny sample of what is really a big scientific field.
Fillingim RB, Gear RW (2004) Sex differences in opioid analgesia: clinical and experimental findings. European journal of pain 8:413–425.
Greenspan JD, Craft RM, LeResche L, Arendt-Nielsen L, Berkley KJ, Fillingim RB, Gold MS, Holdcroft A, Lautenbacher S, Mayer EA, Mogil JS, Murphy AZ, Traub RJ (2007) Studying sex and gender differences in pain and analgesia: a consensus report. Pain 132 Suppl 1:S26–45.
Lee CW-S, Ho I-K (2013) Sex differences in opioid analgesia and addiction: interactions among opioid receptors and estrogen receptors. Molecular Pain 9:1–10.
Li L, Fan X, Warner M, Xu XJ, Gustafsson JA, Wiesenfeld-Hallin Z (2009) Ablation of estrogen receptor alpha or beta eliminates sex differences in mechanical pain threshold in normal and inflamed mice. Pain 143:37–40.
McCarthy MM, Auger AP, Bale TL, De Vries GJ, Dunn GA, Forger NG, Murray EK, Nugent BM, Schwarz JM, Wilson ME (2009) The Epigenetics of Sex Differences in the Brain. J Neurosci 29:12815–12823.
Sorge RE et al. (2015) Different immune cells mediate mechanical pain hypersensitivity in male and female mice. Nat Neurosci 18:1081–1083.
Wiesenfeld-Hallin Z (2005) Sex differences in pain perception. Gender medicine 2:137–145.
