It’s more about the sources they use. 100% I don’t think the authors realized they were doing this, but most of their assertions about Scots Irish as a cultural group trace back to really nasty ideas about the Irish in the 19th and early 20th centuries, back when the Irish were considered to be kind of on the peripheries of whiteness, and by many to just not be white. Basically English colonial stereotypes of a violent/poor/impulsive Irishperson
I think it’s a situation where because they’re psychologists and not anthropologists or historians, they weren’t totally aware of the antecedents of anti-Irish sentiments and didn’t take that or class stereotypes into account in the way someone from either of those disciplines or say, a sociologist would. It would have benefited greatly from more interdisciplinary consultation or collaboration
There’s about 150 years worth of people trying to prove that Irish people are scientifically more violent and irrational than other European groups that’s very easy to find. It’s a clear intellectual antecedent to this research, which as I said, hasn’t been replicated, because it’s based in stereotypes rather than actual behavior