Child-born neurological disorders vs. early childhood trauma during infancy

How can scientists (psychologists, psychiatrists, neuroscientists) assert the difference between an infant-born neurological disorder (such as brain activity/dispositions that highly correlate to psychopathy, callous-unemotional traits, traits affiliated with any other personality disorder, traits affiliated with PTSD, etc.) and neurological injuries stemming from early childhood trauma during infancy? Were such infant-born neurological deficits actually hereditary, or could intergenerational traumas be the source of neurological injuries post infant trauma?

Were the studies on infants conducted right at birth, or were they conducted sometime thereafter (say, at 6 months), when that time span could have included unsubstantiated maltreatment and/or unsubstantiated non-maltreatment trauma during infancy? You cannot ask an infant such questions, and third-party questions may be biased, so as to avoid child welfare system involvement or other punitive consequences that stem from disclosing early childhood traumas.

How can scientists assert the same in retrospective studies on adults, whom they presume to have a "dispositional character flaw" that they were born with, as opposed to early childhood trauma during infancy that affected infants' brain development, not the other way around?

These are the questions that should be asked, especially in terms of prevention (of course, with the proper lexicon for such terms that I've described).