Common traits trained in new police officers — like conformity and cohesion — have been linked to a strong in-group bias. Cops are trained to act as a unit, making it more likely that they prejudice an outgroup, especially in chaotic situations like a protest.

When one cop acts violently, training and tactics will influence other cops to react violently as well. The “bad apples” can spoil an entire barrel when police act as a unit, not as individuals.

The dirty little secret is that, for the police, high intelligence applicants are screened out of the process. This is done on the theory that highly intelligent people will quickly grow bored with the job and leave well before the costly training is amortized. A translation of that theory is the people who are better able to think for themselves are not wanted in the PD. Despite being ridiculous this is how it is done for many many municipalities in the country.

Conformity is re-inforced if the recruits are all average or slightly-above-average intelligence. It is re-inforced even more if the training officers are all average or slightly-above-average: it’s not quite the blind leading the blind, but rather the unimaginative leading the unimaginative.