Do you have proof it makes them worse?
I tried googling the issue, but most articles I read, especially negative, seem to be pure speculation.
Here's an article I found that does mention research, and suggests that diversity is beneficial.
If you have any research which shows that diversity hurts, feel free to post.
You aren't going to find much on Google. Unless you have access to something like JSTOR or other databases, you will only get nonsense on "managing" diversity or, if you are a little more adventurous, blogs detailing either the unmitigated virtue or evil of the whole thing.
My theory is based on history and documented group behavior patterns. In humans, different groups have different identities and cultures. When these groups are forced to compete with one another for the same resources, conflict necessarily emerges. This is one of the reasons humans self-segregate into different nations, social classes, and so on. Simply put, two groups cannot play the same game when they have different views on what the rules should be.
If you're interested in studying the data that leads to this conclusion, Amazon has plenty of books that discuss the issue either directly or through the lens of nationalism. Alternatively, if you have a history book on hand, simply turn to a chapter of your choosing and note the correlation between diversity and conflict.
I think the point davislane1 is making (apologies if I'm in error) is that
forced diversity can be counterproductive. Any system that awards position based on anything other than merit can bring with it any number of biases.
My quick Google since my own experience is obviously anecdotal
No, I'm saying that forcing the issue
is counterproductive. Diversity is a fundamentally unworkable system. Real differences exist between races, ethnicities, social classes, and genders. These differences span all the way from genetics to culture and philosophy. Consequently, the more diverse a population is, the more it is prone to conflict. The only instances where diversity is workable are when that diversity is, from a practical standpoint, illusory. That is, when the differences between two parties are insignificant in comparison to their similarities.
Forced diversity merely adds fuel to the fire. Instead of allowing people to associate freely with one another, it forces them to accept members into their group that do not qualitatively or quantitatively belong there. This not only fails to integrate out-group members, it magnifies the animosity directed to them by the in-group. That animosity is then felt and (usually) reciprocated by the out-member, making them even more unpleasant to the in-group. Once the feedback loop is established, it continues until someone throws a punch. For instance, imagine if there were a new mandate requiring each NBA team to play a midget (a quantitative mismatch) for at least 15 minutes per game, in the playoffs, because equality. There would be riots.