It does not require that scientists be unbiased

From Jonathan Rauch, In Defense of Prejudice:

"One of the strengths of science," writes the philosopher of science David L. Hull, "is that it does not require that scientists be unbiased, only that different scientists have different biases." Another dirty secret is that, no less than the rest of us, scientists can be dogmatic and pigheaded. "Although this pigheadedness often damages the careers of individual scientists," says Hull, "it is beneficial for the manifest goal of science," which relies on people to invest years in their ideas and defend them passionately. And the dirtiest secret of all, if you believe in the antiseptic popular view of science, is that this most ostensibly rational of enterprises depends on the most irrational of motives--ambition, narcissism, animus, even revenge. "Scientists acknowledge that among their motivations are natural curiosity, the love of truth, and the desire to help humanity, but other inducements exist as well, and one of them is to 'get that son of a bitch,'" says Hull. "Time and again, scientists whom I interviewed described the powerful spur that 'showing that son of a bitch' supplied to their own research."