Personal bias, wishful thinking plus biased research results is one recipe for a sacred cow. If more rigorous research results in a conflicting message, it could cause cognitive dissonance - and the less biased research often faces an uphill battle for acceptance.
And I also wrote about the importance of being just as rigorous about the claims we want to believe as those we're skeptical of here at the British Journal of Sports Medicine blog.
If you want to get better at critically assessing health claims, Smart Health Choices is a great place to start.
In 2018, the Centre for Evidence-Based Medicine at Oxford University began a Catalogue of Bias - with a blog and Twitter.
[Updated on 11 March 2018]