They also tested that male and female betta's who were left with their father longer (meaning he wasn't removed right after they were free swimming) were less aggressive during courting. Thought this was all very interesting stuff.
Wild splendens are not as aggressive/vicious as domestic. Isolation will make them aggressive. I often keep my batch together until they die, either with father or on their own. An adult male will keep fights to a minimum, but will not stop it completely. These will never be as aggressive as the ones I jar/isolate. They will flare and fight, but only to a few bites. Only few will be too vicious to be in a sorority. Unlike the isolated ones, that will fight to the bitter end. I do agree that splendens are the most aggressive out of all the Betta group.
IMO/IME color also determine aggressiveness. Light colors are often less aggressive . . . sorry has weaker mentality
compared to dark colors. The strongest of all is the wild color (a mixture of black and green/turquoise). Light colors will fight but stress easier than dark colors. This is why lighter colors tend to grow slower than darker colors, though not always. IME the lower mentality in dark colors are Blue and red. I'm not sure if this is still true since this species have been excessively mixed bred. But you won't find a light colored "fighter" until this day.
Deformities do happen regardless how you breed them. But these defects can be minimized. As far as I know OFL has succeeded in minimizing, if not eliminating DT defects. She can breed her DT together with no problem. SO if everyone breeds for Health, it can eventually be eliminated or at least minimized.
The problem is that new breeders often don't know what causes certain defects and will breed them, making it worse. They then distribute these to the local market and spread them. . . . this is also a reason why many don't advise breeding LFS bettas.
Known breeders will always try to avoid breeding unhealthy pairs and will cull the whole batch including the parents. Though they inbreed and sometimes try to make fins larger or thicker, they know what they're doing and will take necessary measures to ensure that their line/s remain healthy. I've heard some claim that their rose line are safe to breed. . . . in fact I've seen someone (forgot who) here breed (I think) Karen's RT and didn't get any deformed fry.