As a geek, I find the Evolutionary Psychology theories about prejudice quite plausible:
Researchers in the field have hypothesised that we may be pre-disposed to crudely classify people, animals and plants into simplified groups - e.g. dangerous / friendly / delicious with onions - as a survival measure.
However, what helped primitive hunter gatherers on the Savannah Plain 200,000 years ago can be disastrous in our highly complex society, if applied without thinking properly.
Presumably, any inherent tendencies would be reinforced or reduced by the environment of the surrounding society, plus individual life experiences.
Importantly, survival instincts kick in hard when we feel ourselves, or especially our families, are in danger and these can override the reasoned opinions we would normally have.
So, we need to keep reminding ourselves not to lump together a diverse group of people as “the other”, whether on ground of religion, race, politics, body shape.
It’s not being patronising: we are all susceptible to this tendency and noone should feel either smugly superior or unfairly picked on.
We must consciously train ourselves - not just others - and be watchful to avoid unfair / superficial classifications.