Here’s a rock-solid argument against racism, sexism, xenophobia, homophobia, or any other form of prejudice against an out-group:

People are wrong to believe that there are important differences between us and them.  We should treat the out-group the same as we treat the in-group, because the out-group and the in-group are, on average, the same.

Thus, the cleanest argument against race-based employment discrimination is, “Workers of all races are equally productive.”  The cleanest argument against single-sex clubs is, “Men and women have the same personalities and interests.”  The cleanest argument against national origins immigration quotas is, “People from all countries make equally good citizens.”  The cleanest argument for legal gay marriage is, “Marriage is just as worthwhile for gays as it is for straights.”  And so on.

This does not imply, of course, that group differences automatically justify unequal treatment.  But group differences plainly complicate matters.  As group differences grow, the case for equal treatment becomes more open to doubt.  Almost no one complains when 90-year-olds have to renew their driver’s licenses more frequently than 40-year-olds, because the average gap in driving ability between these two age groups is vast.

So suppose you want to discourage prejudice.  Anti-discrimination activists usually focus on people’s perceptions about group differences.  And if people overestimate group differences, fixing perceptions is indeed the obvious path.

There is, however, another approach.  Instead of trying to change what people think about group differences, you could try to shrink those differences.  Though extreme bigots may cling to their negative views, the straightforward way to change what people think is to change what they see.

Now here’s what’s weird: People who detest prejudice often do the opposite.  Instead of trying to make differences go away, they accentuate them.  How?  By building group identity.  Instead of saying, “We’re the same as you, so stop treating us badly,” activists usually prefer to say, “We’re proud of our distinctive culture and attitudes, so stop treating us badly.”

Think about the dynamics.  When the activists’ people hear this message, what’s their natural reaction?  To make their identity even more pronounced.  After all, it’s a great source of pride, so let’s double down.  When outsiders hear this message, similarly, their natural reaction is not to stop discriminating, but to dwell on this declaration of difference.  If you’re so proud to be different from us, they silently ask, why shouldn’t we treat you differently?  From here, a vicious spiral of pride and prejudice ensues.

Now, you could object, “If you’d treated us well all along, we wouldn’t have sought solace in our group identity.”  But whoever you blame, the fact remains: Identity causes prejudice.  This doesn’t mean it’s the sole cause of prejudice.  But is is one important cause.  And you can do something about it.

The lesson: If you want prejudice to go away, don’t tell people to be proud of their identities.  Instead, ask them to focus on our common humanity – the sense that our differences are “only skin-deep.”  If you must have a sense of identity, make it not proud but casual.  Despite what you’ve heard, appeasement usually works.  When I describe myself as “an openly nerdy man,” both nerds and non-nerds laugh.  No one worries about wounding my pride as nerd.   And that’s just the way I like it.