i think it can come from many minor incidents, like rolling their eyes, tutting, sighing etc, rather that big acts of cruelty.
when someone is openly hostile, people get defensive and put up a barrier, but when someone 'niggles' in small ways, your walls are down and over time it can instill a sense of not being good enough.
if this happens with a child and their parents, the child is likely to develop 'issues' in later life.
many adult don't realise that what, to them, is nothing, and that they don't mean anything by it, or thinks it just showinga bit of irritation, can seem very different to a small child and create emotions they don't understand.
i think adults should be forced to undergo classes in child care - but not just physical safety and care, but mental stimulation and care too.
there are too many facked up people in the world and so often it turns out to be their upbringing.