Being a parent is something generally learned from your parents (based on how you were raised), with maybe a bit of advice from books/tv/friends. A parent with a traumatic past/upbringing can easily bring that into the situation of the present with their raising of their own children, effectively "passing it on" to a new generation. These "lessons" can be very subtle because like many things learned as very young children (a lot of that through mimicry and observing your parents), people tend to take them for granted without question (until something comes up to make you consciously question it).
For example, the people that think it's fine to hit their children or yell at them because "that's what their parents did." Parenting is one of those seemingly crucial skills that is hard to learn, doesn't take a license, everyone is thrown into without practice for their first child, and can radically affect how children grow and develop.
This of course is both personal anecdata, having talked to a lot of people, and reading a fair amount of books on childhood trauma. It's a hard field to get hard scientific research on, since it's not really ethical to do that kind of research on children, but here's a recent paper of interest:
https://www.sciencedaily.com/releases/2018/07/180709101155.h...
"Parents who lived through adverse childhood experiences were more likely to report higher levels of aggravation as parents and to experience mental health problems, the researchers found. However, these mental health and attitude factors only explained about a quarter of the association to their child's elevated behavioral health risks. The remainder of how the parent's adverse childhood experiences are transmitted to their child's behavior deserves further study."
So maybe epigenetics are one of these other ways things get transmitted. But I think it's a lot easier to study because it's easier to measure than something like childhood experience. Even children can't be expected to remember what really happened.
> Even short-term substance abuse can produce long-lasting epigenetic changes in the brain of rodents, via DNA methylation and histone modification.
The principle here is straight forward, but it's not clear to me that anybody has actually demonstrated a clear 'signal' in the process.
In other words there is evidence that stress caused by fire trauma can change the way genes are expressed in an individual, and I believe there is evidence that stress from trauma can impact gene expression at the process of genetic recombination. However I've not seen any evidence that, for instance, stress caused by fire will induce any sort of specifically fire related gene expression in offspring. Would an orphan who never knew their parents have an elevated fear of fire if both their parents had survived a severely traumatic experience with fire? To my knowledge, that's still purely speculative.
The closest I've seen is the suggestion that offspring of famine survivors may have an epigentic change that makes them gain weight:
>The field of epigenetics gained momentum about a decade ago, when scientists reported that children who were exposed in the womb to the Dutch Hunger Winter, a period of famine toward the end of World War II, carried a particular chemical mark, or epigenetic signature, on one of their genes. The researchers later linked that finding to differences in the children’s health later in life, including higher-than-average body mass.
But that leaves open many questions. What other epigentic changes did these people have that seem unrelated to food/weight? And how many people who suffered from trauma unrelated to food availability have changes that effect weight gain?
The big and controversial step is to writing such changes into the germ-line, so that offspring of the individual cary such a memory. And my understanding is like yours, that there's some evidence for crude things (like massively stressed / starved mice having offspring who are calibrated a bit differently) but not really anything solid for the sort of finer changes we like to speculate about in humans.
It's not impossible, and it need not be un-Darwinian: your genes care more about how many grandkids you have than how many kids, and if pre-adjusting your kids to the environment is advantageous, then they certainly have the motive to do it. And there are corners of biology in which you could hide a mechanism to do this (just a little bit) where we would not yet have seen it.
But a lot of the tantalising studies seem to underplay ordinary inheritance. For example, lots of abused children, sadly, have a 1st-degree relative who abuses children. Some of whatever is wrong with the abuser may also be wrong with the victim (through no fault of anyone's!) and this may show up on all sorts of measures. Removing such things from your data is going to be super-hard, and if you succeed, you are more likely to get a boring paper.
Good point, there's an ocean of difference between experiencing hunger in the womb, and being well nourished in a womb belonging to a woman who was once malnourished.
I don't have that study available to me at the moment to check, but I wonder what they say about children born from those same mothers years after the famine was over. Surely they looked at that too, since if the same signal was present that would certainly enhance their findings. If the study doesn't mention those later children at all, that would suggest to me they're guilty of p-hacking.
But I can well imagine that, in most cases, trauma leaves a body permanently altered in some manner and that this can potentially impact later children.
With no real scientific basis to back it up, I will suggest that if trauma is passed on to the children biologically, the degree and manner of the inheritance may vary depending upon whether the traumatized parent was the father or the mother. Logically, the fact that the biological mother (in "old fashioned" pregnancies) both contributes DNA and also serves as the incubator for the fist nine months of life, whereas the father only contributes DNA, should make a difference in how heritable such a thing is.
I don't know whether anyone has looked at this in mice, I mean in the torture-the-parents experiments which they do to look for epigenetic inheritance.