Whether the tendency towards becoming emotionally dysregulated is something purely genetic in origin or is the result of environmental factors in genetically-vulnerable individuals is an open question. Clearly both genes and environment contribute to most personality traits, but how much of each is required? I will mention some evidence for the answer to this question later in this post.

The second causal factor in the genesis of BPD, according to Linehan, is what she refers to as an invalidating environment.

Invalidation, as used in psychology, is not merely people disagreeing with something that another person said. It is rather a process in which individuals communicate to another person that the opinions and emotions of that person are meaningless, irrational, selfish, uncaring, stupid, most likely insane, and wrong, wrong, wrong.

Invalidators let it be known directly or indirectly that their target's views and feelings do not count for anything to anybody at any time or in any way. In some families, the invalidation becomes extreme, leading to physical abuse and even murder. However, invalidation can also be accomplished by verbal manipulations that invalidate in ways both subtle and confusing. 

Dr. Linehan wrote only briefly in her book (Cognitive-Behavioral Treatment of Borderline Personality Disorder) about which environment she is talking about as being invalidating (page 56-59), and she barely mentions it in her talks and videos. It is the family environment in which the person grew up.  Really, what else could it be? Of course, your spouse and friends can also invalidate you, but why would you choose to fall in with an unpleasant group like that if you were not already accustomed to this sort of treatment?  

When it comes to DBT, however, most of the energy in the psychotherapy treatment described by Dr. Linehan is directed at helping the patients accept themselves as they are, without much said about how they got that way in the first place, combined with teaching patients other skills that are helpful in reducing their emotional reactivity. These are referred to sometimes as self-soothing skills, and are presumed by many DBT therapists to be something that patients with the disorder just never learned.

I sometimes give my patients with BPD handouts which describe these skills from Dr. Linehan's Skills Training Manual. I usually find that patients have already tried at least some of these techniques on themselves without having had any instruction at all. That makes me wonder if perhaps their apparent lack of knowledge about the techniques in some interpersonal environments actually reflects a strong, fear-induced desire not to use such skills, rather than an actual absence of them.

In her book, Dr. Linehan does say that she focuses on the patient's interpersonal skills later in the therapy process. She even mentions that family therapy might be included. Mentions it once or twice. The first time on page 420 of her book. She does not say anything about what that family therapy might entail.