I say both. I know of a LOT of pitties that are good with other dogs there whole life, as where some can not be around other dogs at all.. Whether there genes are great or not. When someone gets one, have the impression that at about age 3.. Things sometimes change! No matter HOW Much you socialize them... or how you raise them. They were bred to be great Family pets however... if you have a Human aggressive one.. that is the owners fault more or less. This is IMO Of course. Pittie Stuff is always soo 2 sided though. :-)