In developing countries especially, many parents beat their children daily. Many husbands beat their wives daily. Many teachers beat their students daily. In traffic, motorists and touts engage in fights daily. Governments routinely have their citizens arrested, tortured, and killed. Armed criminals rob, rape and murder people daily. Most babies have their private parts mutilated.
All or most of the above would be classified as in-group violence by an objective observer, so it is difficult to agree with you that our ingroup violence as a species is much lower.
Unless you have some source, I think the "developing countries especially" affirmation is unfounded. This happens too frequently in all countries, developed or not.
If you are right, that makes my point even stronger, doesn't it?
Do you think I am being downvoted for saying we have a lot of ingroup violence or for suggesting there is more ingroup violence in developing countries? These seem to be uncontroversial points.
I live in Nigeria. Children get beaten here. From their media, I know it's the same in other African countries, India, etc. Everyone accepts it and we are proud of it. But I know that in some developed countries you can be reported to child services for such. So I was careful not to imply that it's the same everywhere.
Child protection services are a deterrent, yes, but not as strong as one would think. Children usually just endure violence because they think it's normal, or they know child protection services will break their family apart. Abusive parents typically are denounced by other adults, like doctors or teachers.
Still, according to page 172 of this report, it seems you're right, because UNICEF puts more effort at developing countries, although their scope covers much more than just domestic violence: https://www.unicef.org/publications/index_74865.html
Men who beat their wife daily are not majority in any culture.
You can walk into a village or bar full of strangers and you don't automatically get into a fights. You don't have to maintain your position with constant violence.
I doubt there is human culture where average adult gets into violent confrontation for status monthly.
All or most of the above would be classified as in-group violence by an objective observer, so it is difficult to agree with you that our ingroup violence as a species is much lower.