There is several contradicting data points for what is an optimal hiring strategy for a company. We have the old strategy of making members of a team feel like then are kin, a.k.a band of brothers. The military use/used this a lot and the underlying biological theory is that people will sacrifice self-interest for individuals which is perceived as kin, or as the quote goes a person will sacrifice themselves for either two siblings or eight cousins.

The opposite strategy is diversity with the ability for better adaptation. A team with multiple perspectives is said to better anticipate the need of a diverse customer base.

A third strategy is to look at specific risk. If for example theft by employees are a high risk issue then rejecting applicants with correlating traits with high risk for theft would be beneficial, which would be low social economic status, but those correlate to race and ethnic class.

If we look at the issue from the perspective of what benefit a company the most we end up in many cases with massive discrimination. Thus for social reasons we address the issue as an ethical question.