If ever I run out of creationist pseudoscience (it will never happen), I can always turn to another source, the Men’s Rights movement, especially their radical anti-woman wing. Here’s a prime example from RooshV: Research Suggests That A Woman’s Body Incorporates DNA From The Semen Of Her Casual Sex Partners. Would you be surprised if I told you that everything in that title is wrong? Would you be shocked to learn that everything Roosh concludes from misreading that research is also wrong?

The above study has two seismic implications. The first is that a woman can absorb enough DNA during her lifetime that it changes her phenotype (i.e. her appearance and overall health state). There could be some truth to the phrase “slut face” in which highly promiscuous women suffer a change to their appearance because of all the variable sperm from different males that have been deposited inside them. The second implication stems from the fact that it’s scientifically conclusive that single mothers have DNA of their bastard children residing permanently within their bodies. Any man who reproduces with a single mom will have a child that contains DNA from the bastard spawn, which of course includes DNA from the absentee father. This means that men can be genetically cuckolded without being traditionally cuckolded, and that having a baby with a single mom is essentially giving the father of her first child a bonus prize in the game of evolution.

There’s literally nothing correct in any of that mess. Nothing. Roosh has imposed his faulty, biased interpretation on the work in a way that would certainly horrify the authors.

The original research studies microchimerism, the incorporation of a small number of cells of a different genetic background into an individual. This happens! It’s been repeatedly confirmed. There is no fusion of DNA into the host, however; what happens is that cells from a foreign individual get infused into the host, tumble about, and take up transient residence throughout the body. It’s been noted in blood transfusions into people with severe trauma — some of the donor blood cells infiltrate tissues through lesions. It also happens during pregnancy. Pregnancy involves very intimate, long-term association between the fetal placenta and the maternal endometrium — those tissues interdigitate and basically fuse together, nutrients and blood flow between them, and some cells also migrate between the two genetically different humans.

This exchange is relatively easily observed (“relatively”: it involves examining the tissues of women who died during pregnancy for fetal markers, which are very thinly scattered). One of the easiest markers to look for is the Y chromosome: the mother’s cells lack it, but any cells derived from a male fetus will carry it, so you just scan through lots and lots of cells, counting Y chromosomes.

The answer is that there are some, but they are very thin on the ground. One study found that less than 0.05% of the cells in the lungs were microchimeric, fetal cells…and the lungs, with their dense capillary beds, were the richest source. Most other tissues had significantly less than 0.01% fetal cells. Note that these measurements were made in pregnant women, when there is a continuous influx of fetal cells, and so is probably the highest frequency that would be observed. Male microchimeras have been found in much older, non-pregnant women, so some of these cells can persist for a lifetime, but I would expect the numbers to drop rapidly after pregnancy.

Also, semen is highly unlikely to be a source of microchimeras. Sperm are highly specialized cells that don’t live long — they’re entirely gone within days. The few other cell types (for instance, white blood cells) are going to be far lower in dose than what you get from pregnancy. It’s simply silly to suggest that there is significant incorporation of your sex partner’s cells.

Given the tiny numbers and the insignificant contribution of microchimeric cells, the idea that semen modifies women’s appearance to give them “slut face”, whatever that is, isn’t just stupid — it’s daft. It’s also rather insulting to males: essentially 100% of the cells in our faces have Y chromosomes. We must have really slutty faces.

So no, his first claim is absolutely false. Women do not absorb enough DNA during her lifetime that it changes her phenotype — there is nothing in any of the papers to suggest such a thing. For one thing, they don’t absorb DNA — they are colonized by a minuscule number of fetal cells during pregnancy. These cells represent a tiny fraction of the total, and no, they do not make a genetic contribution to change her phenotype.

The second claim is messier, but also false. There have been observations of second generation microchimeras in fetuses — that is, the mother contains a tiny fraction of microchimeric cells from a previous pregnancy, and a tiny fraction of those cross into the placenta of the fetus, and an even tinier fraction of those can get incorporated into fetal tissues. (I suppose it’s also likely that you could have third generation microchimeras, but there’s a tinier and tinier fraction of a fraction in each generation). So yes, if your partner had been pregnant by another male before, any subsequent pregnancies might incorporate a vanishingly small number of the previous male’s cells in the fetus.

However, this isn’t an example of any kind of genetic freeloading. If anything, it means that a few of those previous male’s cells might get enslaved to contribute to the somatic tissues of your child: the germ line, the cells that produce the gonads of the child and are going to produce your grandchildren, are set aside very early in development and are not going to incorporate the microchimeras, especially since the microchimeras consist of primarily previously differentiated cells that are not in the totipotent state of a germ cell. There is no genetic cuckolding going on. I just have to roll my eyes at this paranoia of MRAs.

It’s also not going to make any kind of evolutionary contribution.

So relax. Biology is messy. There are cells of all sorts slopping about, especially during sex and pregnancy — I think the exchange of cells from your microbiomes almost certainly has a bigger effect on your offspring than a few rare microchimeric cells. And no, the cells in semen don’t invade women and turn them into sluts, and no, your former partner’s cells aren’t suddenly going to rise up and impregnate you, months or years after a pregnancy.

Rijnink EC, Penning ME, Wolterbeek R, Wilhelmus S, Zandbergen M, van Duinen SG, Schutte J, Bruijn JA, Bajema IM. (2015) Tissue microchimerism is increased during pregnancy: a human autopsy study. Mol Hum Reprod.[Epub ahead of print]

Chan WF, Gurnot C, Montine TJ, Sonnen JA, Guthrie KA, Nelson JL. (2012) Male microchimerism in the human female brain. PLoS One. 7(9):e45592.