A Peaceful Society Full of "Identical" Twins
Society improves drastically when everyone is given an "identical" twin of the opposite sex. In utero, chromosome 23 is changed but everything else is duplicated exactly.
Growing up with an opposite-sex twin helps each person temper the more extreme parts of their personality that come from either nature or nurture of their sex.
There have been no wars and violent crime is down significantly. All institutions are significantly more just and fair. There is much less loneliness and, consequently, many fewer suicides.
However, when things go bad, they really go bad. Certain twins team up to go on crime sprees or partner up in white-collar crime schemes.