Not that all peer effects are so bad. Shop workers become more productive when they work alongside more productive colleagues, a finding corroborated by this study (pdf) of fruit-pickers.
And then there’s a ton of evidence for peer effects in schooling. Some of it is summarized in this pdf. Other papers are here, here, here (doc) and here (pdf). The most notorious paper is Roland Fryer's claim (pdf) that negative peer pressure ("acting white") can hold back able black students in school.
You might object here that such peer effects are over-estimated, simply because like attracts like. Maybe teenagers who take drugs choose other friends who take drugs, so what looks like a peer effect is in fact a mundane selection effect. However, many of the papers overcome this problem by using random selection or other controls. And this paper finds that peer effects work even in laboratory settings.
So Danny’s right - peer effects matter.
This is not, however, entirely welcome for traditional conservatives, for three reasons. First, because it complicates the question of moral responsibility; if individuals‘ life outcomes are so heavily influenced by the accidental circumstances of whom they associate with, it‘s difficult to hold them responsible for what they become in later life - as I argued here.
Second, it’s an argument against selection in education - by ability or income, because such selection deprives less able children from poor homes of the opportunity to benefit from positive peer effects.
Thirdly, it’s an argument against banging up young criminals. After all, if peer effects matter, surrounding someone with criminals will create more crime, as this pdf shows.