When Robert Putnam runs a proper statistical horserace, one of his favorite predictors of trust – diversity – barely matters.  To recap:


Now a diversity skeptic could look at Putnam’s results and say: “Fine, diversity per se is no big deal.  But Putnam does show that blacks and Hispanics have low trust.  And that’s controlling for household income, the area’s poverty rate, and Spanish prevalence, all of which further depress trust.  The presence of blacks and Hispanics is truly terrible.”

The easiest reply is: “You’re right qualitatively, but not quantitatively.”  The whole point of a regression is to measure the size of effects, not just their directions – and the size of demographic effects is modest.  Suppose the black share rises from 5% to 55%, the poverty rate rises from 10% to 40%, and average household income falls from $75k to $25k.  This drastic demographic shift reduces Putnam’s predicted trust by .31*.50 + .66*.5 + .5*.14 = .56.  Is that massive?  No, because Putnam measures trust on a 4-point scale.  .56 is less than 20% of size of the trust spectrum – noticeable, but hardly the end of the world.

But there’s a subtler reply.  Namely: The effect of demographics on trust is zero-sum.  If low-trust people move into a high-trust area, the change is bad for the incumbents but good for the entrants.  Calling black migration “bad for trust” is just NIMBYism: keeping low trust away from you doesn’t make society’s trust higher.

Isn’t this always true?  No.  If, as in Putnam’s original story, diversity per se were really bad for growth, segregation would sharply raise average trust.  Indeed, segregating two communities could conceivably raise trust in both communities.  This is what makes diversity a special social variable.  If diversity in and of itself has bad effects, so does integration – regardless of the characteristics of the mingling populations.  If the effects of diversity are demographic effects in disguise, however, integration has distributional effects, but is zero-sum overall.

In a sense, then, Putnam was right to focus on diversity, because diversity is conceptually special.  In the real world, arguments about diversity usually boil down to identity politics: Diversity is bad if it hurts my group, good if it helps my group.  In theory, however, you could have an anti-diversity universalist – someone who thinks that society as a whole will be better off if people stick to their own kind.  Putnam’s empirics suggest that anti-diversity universalists are rare for a reason: The numbers just don’t add up.