Gunderson, one of the leading researchers of BPD, said in a conference (I can't remember who the other panelists were) that they have no empirical evidence that birth order influences BPD.
He also talked about schizophrenia, which has a very high rate of heritability (like 90 percent), but that for some people, the heritability is 100 percent, for others it is less. Heritability in BPD can apparently be the same. It can be 100 percent heritable, and it can also be 40% and 25% and so forth. In Aguirre's book, I believe he says the most recent empirical studies suggest an average of 60% heritability and 40% environment. Meaning, of everyone diagnosed BPD, 60% are likely to be BPD due to nature, and 40% due to environment.
There is another interesting thing that never occurred to me, which I am still trying to sort out, that we often think that parents shape the child. Gunderson says that there seems to be an opposite thing going on, where the children are shaping the parents. Particularly when there are high-maintenance children (i.e. difficult to soothe) and caregivers who don't have a "goodness of fit" in terms of temperament. He mentioned that a highly sensitive child might have a very difficult time with an emotionally reactive caregiver, whereas the same child might have an easier time with a parent who was less reactive. It reminds me of threshold theory with migraines, that there is no one trigger, but a bunch of things that push someone closer to the threshold where migraines then occur.
In my situation, I think my son has some mild to moderate sensory processing issues and a sensitive genotype (highly sensitive child or HCP), which was mitigated by my attentiveness, exacerbated by not understanding his sensory distress, and then devastated by having a highly invalidating and alcoholic N/BPD father who modeled maladaptive coping techniques. Emotions were suppressed in my family of origin, and when I gave birth to a child with "big feelings," I did the best considering what I knew. Looking back, knowing what I know now, I can see how my son needed help describing his emotional state. My tendency was to soothe him by trying to dial down his feelings, which was more familiar to me. When I think of what Gunderson says about the child shaping the parent, that has been the case for me. After all these years, I am now bringing to our relationship what he needs. He is shaping me, and this has been the greatest period of growth in my life as a result.
I feel that learning this did alleviate some guilt for me, and brought many aha moments -- although, like lbjnltx, I do think focusing on the cause can take our efforts off what is most helpful, which is to learn the communication skills and other techniques that can help us help ourselves, and our kids. For me, the
moment is recognizing that despite my nurturing, loving, attentive parenting, I was slow to recognize how high my son's needs were to name and label his emotional states, and by extension, my own too. Gunderson talks about how people with BPD seem to know what others are thinking before we do -- this is perhaps the blessing and the curse of being so emotionally attuned to the moods and feelings of others. They are reading us, and we are not always reading them, and so there is a lot of potential for mutual learning here, as difficult as these lessons can be.
