> Humans are always going to prefer people similar to > themselves, so you need to make everyone similar.

A very strong and potentially pernicious conclusion for such vague premises.

Wouldn't it be more accurate to say that humans feel reassured within their group(s) (whatever that is) and also sometime feel threatened by other groups? This behavior itself being worsened by social insecurity (giving signs of one's belonging to the in group by being hostile to the out group), as frequently documented.

Following this line of thoughts I don't think looking more similar would solve anything. It's not aiming at the problem and it's not even doable (we would have to look the same, speak the same, think the same, etc, or we would always be able to form groups).