But those are just numbers, which mean both a great deal and nothing at all when it comes to determining why any sibling pair will commit a collaborative crime. In the current case, there were a few factors that probably nudged those risk figures higher. For one thing, the young men lived together. Proximity is a critical element in what psychologists call “delinquency training” among siblings. This is true for lower-grade risk behaviors like smoking or drinking, since a younger sibling, who typically picks up the habit later than an older one, must be able to observe and model the bad behavior—to say nothing of getting hold of the forbidden substances in the first place.

Psychologist Elizabeth Stormshak of the University of Oregon conducted a study in which sibling pairs were interviewed on videotape along with other kids and were asked to pretend they were planning a party and that some kids were going to bring drugs. They were then told to debate whether that was alright and whether they might want to try them themselves. In general, the likelihood of a younger sibling’s saying that that was indeed OK was directly linked to whether an older sibling had already said the same. The direct mimicry would not have been possible without direct proximity.

It’s a lot easier to pick up a cigarette or a pill than to pick up criminality, of course, but when adult siblings share a home, the influence one exerts on another can be hot-housed. Researchers believe that siblings who live together as young adults—which is often a sensible and perfectly healthy arrangement, particularly in tough economic times—will ultimately hit a developmental ceiling at which their continuing total-immersion contact with a member of their original nuclear family begins to stunt their social growth. Consciously aware of that or not, they tend to separate and enter what psychologists call a sibling moratorium phase, spending far less time with each other until they’ve settled into their own adult lives. Hanging around together too long prevents that developmental step.