What scientific evidence can you produce to dispute that the cause of a certain behavior is the result of a traumatic childhood experience?How to...

I'm not sure that without some sort of medical or psychological
degree, you have it in your power to disprove your friend "scientifically."
In fact, when it comes to behavior, psychology is unique in that most of it is based on
"theories" and some seem to work better than others in different
situations.

Blaming a behavior on a traumatic childhood experience
is a very Freudian approach to behavioral analysis (Sigmund Freud). Some
more modern psychologists have disputed the idea that identifying and/or blaming behavior on
something that is outside of our control is the best way to produce
change.

One such psychologist is William Glasser, who proposes in
his "Choice Theory" and "Reality Therapy" that despite what a person has gone through or
experienced in the past, it is through his desire to change his future that
will allow him to change his behavior. Glasser basically had patients describe their "Quality
World Picture," which is essentially a goal for the distant or immediate future (where they one
day see themselves or something they wish to achieve). Then, he helped them outline the shortest
steps they needed to take to reach that goal. The best part of his therapy was that it was based
on his relationship with his patients and therefore was very personal. He was able to counsel
individuals into seeing that their behavior is up to themselves, and though outside factors may
affect their environment, negative factors do not have to negatively affect
their choices.

Reality therapy basically looks
at the positive consequences of appropriate choices and behavior (staying on track to reach a
goal) and the negative consequences of negative choices (behavior which would not result in
reaching the goal). By reminding patients of what they want and showing them how to get
themselves there, Glasser experienced great success in behavior
modification.

Modern psychology tends to focus more on the future
rather than the past. Perhaps instead of engaging in a potentially futile argument over the
cause of the behavior, you can engage your friend in a discussion of what to
do next. If this subject interests you, I highly recommend the book Reality
Therapy by Glasser. It is a short read, but immediately applicable when it comes to
behavior change.