Scientific bias is the assumption that a theory is true or false without evidence one way or another, or the attempt to dismiss or discourage research efforts to confirm or deny the theory - often on political or ideological grounds. This is generally seen as an obstacle to applying the scientific method.
A physicist wrote:
- ... Another common mistake is to ignore or rule out data which do not support the hypothesis. Ideally, the experimenter is open to the possibility that the hypothesis is correct or incorrect. Sometimes, however, a scientist may have a strong belief that the hypothesis is true (or false), or feels internal or external pressure to get a specific result. In that case, there may be a psychological tendency to find "something wrong", such as systematic effects, with data which do not support the scientist's expectations, while data which do agree with those expectations may not be checked as carefully. The lesson is that all data must be handled in the same way. 
Another physicist, Richard Feynman, wrote:
- ... if you're doing an experiment, you should report everything that you think might make it invalid-not only what you think is right about it: other causes that could possibly explain your results; and things you thought of that you've eliminated by some other experiment, and how they worked-to make sure the other fellow can tell they have been eliminated.
- Details that could throw doubt on your interpretation must be given, if you know them. You must do the best you can-if you know anything at all wrong, or possibly wrong-to explain it. If you make a theory, for example, and advertise it, or put it out, then you must also put down all the facts that disagree with it, as well as those that agree with it. 
Professor Thomas Gold wrote:
- Once a herd has been established in a subject, it can only be broken by the most crass confrontation with opposing evidence. There is no gentle way that I have ever seen in the history of science where a herd once established has been broken up. 
President Eisenhower warned:
- "The prospect of domination of the nation's scholars by Federal employment, project allocations, and the power of money is ever present and is gravely to be regarded. Yet, in holding scientific research and discovery in respect, as we should, we must also be alert to the equal and opposite danger that public policy could itself become the captive of a scientific technological elite." 
The American Psychological Association, for example, opposes reparative therapy for homosexuals (even if they volunteer for it), although they themselves acknowledge that there's no proof it does any harm:
- ... there are no scientifically rigorous outcome studies to determine either the actual efficacy or harm of "reparative" treatments.
A historian of science and the church writes:
- Scientific methods are not intended to eliminate the researcher's bias (which is impossible), but to neutralize the effect of these biases on the final results of the research.
- ... Pride, fear and power all influence how research funding is allocated.