I don’t think there will ever be a legitimate study.
One of the theory's major protagonists is the respected liberal Harvard professor Robert Putnam, known for his 2000 book "Bowling Alone," in which he uncovers a sobering decline in civic engagement in America. He has now come up with some astonishing findings that are so shocking that he did not release them for several years.The truth, Putnam found, is that the greater the amount of multiculturalism and diversity in a society, the lower the level of civic engagement and shared sense of community cohesiveness. The multiculturalists who have dominated our social thinking for nearly 50 years got it all wrong.