All of you have read on this site (most recently in my critique of a dire New York Review of Books article) about the buzz concerning “epigenetics”—in particular, about the idea that human DNA can be changed by our exposure to the environment, and the view that such DNA changes can be inherited across several generations. Some people claim that this makes possible a form of non-Darwinian evolution, since the hereditary changes are actually caused by the environment, but there’s not a whit of evidence of any adaptation that arose in this way.
And, as I’ve said before, there’s no evidence that environmentally induced changes in DNA can persist beyond a few generations, making this “neo-Lamarckian” evolution very unlikely. Finally, when you actually map evolutionary differences between species, adaptive or otherwise, you invariably find that they map to changes in the DNA sequence, not to changes in “methylation” of DNA bases—the oft-cited source of the “environmental modification” of DNA. What we have is a lot of sizzle and no steak.
A new paper in Wiring the Brain, “Grandma’s trauma: a critical appraisal of the evidence for transgenerational epigenetic inheritance in humans“, by Kevin Mitchell (a professor of developmental neurobiology and genetics at Trinity College, Dublin), does what I never did: minutely scrutinize the several scientific papers that claim to show epigenetic inheritance in humans—usually an effect on grandchildren from starvation or trauma of grandparents. These are the studies widely touted as showing epigenetic modification of DNA by the environment (e.g., via famine) that gets inherited for at least one generation not exposed to the environmental stressor (the grandchildren).
Mitchell shows that every single study claiming this suffers from serious flaws. If you have any interest in epigenetics, his not-too-long article is worth reading. In general, these studies suffer from the following flaws:
1.) There’s no evidence of any epigenetic modification of DNA. For all papers but one, the sole evidence is that grandchildren of stressed grandparents differ in phenotypic traits (health, birth size, etc.) from those of non-stressed grandparents. To be fair, there’s one study showing methylation differences at just five DNA positions in a small sample of 121 people whose grandparents were or were not exposed to violence. But there’s no a priori hypothesis, and p-hacking is a real possibility (see #3).
2.) The sample sizes of humans are small, and the effects are very small.
3.) There seems to be pervasive “p-hacking”: that is, if you do sufficiently many correlations, using different aspects of grandchildren’s health, correlating with grandfather or grandmother, and so on, you’re bound to find at least one effect that is significant but is due not to a real phenomenon, but to chance deviations expected under no effect.
and, as the authors note
4.) “Lack of predefined hypotheses”. That is, they are dredging the data looking for effects.
I might add here this possibility, too:
5.) Publication bias. How many attempts to find “grandparental effects” failed, and thus weren’t published?
It’s time to put to rest, at least for a time, the claim that humans show environmental modification of their DNA that can be passed on to grandchildren. I didn’t read the original papers, so I didn’t really know how weak even these claims were, although one can dismiss the possibility of evolutionary change simply because there’s no cases in which epigenetic changes in DNA last more than a couple of generations.
So why all the buzz about epigenetics? I close by quoting Mitchell’s explanation for why do all these “grandparental famine” studies get so much publicity in the popular press:
. . . . . why do these studies get published and cited in the scientific literature and hyped so much in the popular press? There are a few factors at work, which also apply in many other fields (everything indented is a quote):
- The sociology of peer review. By definition, peer review is done by experts in “the field”. If you are an editor handling a paper on transgenerational epigenetic inheritance in humans (or animals), you’re likely to turn to someone else who has published on the topic to review it. But in this case all the experts in the field are committed to the idea that transgenerational epigenetic inheritance in mammals is a real thing, and are therefore unlikely to question the underlying premise in the process of their review. [To be fair, a similar situation pertains in most fields].
- Citation practices. Most people citing these studies have probably not read the primary papers or looked in detail at the data. They either just cite the headline claim or they recite someone else’s citation, and then others recite that citation, and so on. It shouldn’t be that way, but it is – people are lazy and trust that someone else has done the work to check whether the paper really shows what it claims to show. And that is how weak claims based on spurious findings somehow become established “facts”. Data become lore.
- The media love a sexy story. There’s no doubt that epigenetics is exciting. It challenges “dogma”, it’s got mavericks who buck the scientific establishment, it changes EVERYTHING about what we thought we knew about X, Y and Z, it’s even got your grandmother for goodness sake. This all makes great copy, even if it’s based on shaky science.
- Public appetite. The idea of epigenetic effects resonates strongly among many members of the general public. This is not just because it makes cute stories or is scientifically unexpected. I think it’s because it offers an escape from the spectre of genetic determinism – a spectre that has grown in power as we find more and more “genes for” more and more traits and disorders. Epigenetics seems to reassure (as the headline in TIME magazine put it) that DNA is not your destiny. That you – through the choices you make – can influence your own traits, and even influence those of your children and grandchildren. This is why people like Deepak Chopra have latched onto it, as part of an overall, spiritual idea of self-realisation.
So, there you have it. In my opinion, there is no convincing evidence showing transgenerational epigenetic inheritance in humans. But – for all the sociological reasons listed above – I don’t expect we’ll stop hearing about it any time soon.
When you hear these claims, then, just remember this post—and Mitchell’s analysis.