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 argue with a friend who is trying to claim "traumatic childhood experience."
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.