If you're old enough to be here, you've already seen all the evidence you need.
And not just humans. Pretty easy to tell if an animal has been mistreated. Kittens raised on the street, where they compete with each other for food, are nothing like those born and raised indoors.
Do you really need "hard science" for this kind of stuff?
I don't think it's unreasonable to ask for "hard science" on this. Just as you've provided an anecdote of "street kittens," I have personal anecdotes of children who grew up in abusive families who turned out to be very nice people. Of course, there may have been less visible setbacks in other parts of their development, but at the end of the day it's not a clear-and-cut A causes B to me.
You are right. It's not unreasonable to ask, however it wouldn't hurt to demonstrate some basic acknowledgement that things do seem to be strongly correlated. Even the biggest idiots understand the concept.
You are also right that not everyone (human or different animal) reacts the same. Some turn the other cheek, some will break your nose. The scars will always be there, though.
I've never seen any evidence that changes in gene expression are mostly negative. The entire point is gene expression, not that stress or trauma causes a person to behave differently afterwards. It's weird that people on this thread don't seem to understand this very important distinction.
And not just humans. Pretty easy to tell if an animal has been mistreated. Kittens raised on the street, where they compete with each other for food, are nothing like those born and raised indoors.
Do you really need "hard science" for this kind of stuff?