Definitely the unstable environment & constant unpredictability growing up in my house. And being abused too. Being teased, harrassed, bullied & picked on at school through the years I'm sure contributed as well.

I found the best and most believable research & info on BPD when I was researching childhood sexual abuse actually. There are a lot of theories out there, some of which are completely crack-pot if you ask me, but the ones I read about during the CSA research sounded the most valid to me. I don't buy into the we were born more sensitive than everyone else theory. The ones that explained how a child's brain can be overwhelmed with a saturation of neurochemicals during abuse or trauma that its brain is not yet developed or equipped to handle, & that subsequently leads to a different formation of neural connections than everyone else's, that sounded a bit more legit to me.

And I believe in both nature & nurture by the way, so I definitely am not the type to buy into any theory that claims brain changes for everything. Social environment can mess people up too. I firmly believe in both.