I am answering the current version of the question, which asks: 'Did Robert Putnam publish a study that found that diversity has "incredibly negative consequences"?'
Robert Putnam did publish:
The abstract explains:
Ethnic diversity is increasing in most advanced countries, driven mostly by sharp increases in immigration. In the long run immigration and diversity are likely to have important cultural, economic, fiscal, and developmental benefits. In the short run, however, immigration and ethnic diversity tend to reduce social solidarity and social capital. New evidence from the US suggests that in ethnically diverse neighbourhoods residents of all races tend to ‘hunker down’. Trust (even of one's own race) is lower, altruism and community cooperation rarer, friends fewer. In the long run, however, successful immigrant societies have overcome such fragmentation by creating new, cross-cutting forms of social solidarity and more encompassing identities. Illustrations of becoming comfortable with diversity are drawn from the US military, religious institutions, and earlier waves of American immigration.
He makes it clear that he is still in favour of diversity and immigration, despite these short term challenges:
Increased immigration and diversity are not only inevitable, but over the long run they are also desirable. Ethnic diversity is, on balance, an important social asset, as the history of my own country demonstrates.
In short, while he does show some negative short-term consequences, he does not characterise them as "incredibly negative".