Interpretations of the random patterns of craters on the moon. A common example of a perceptual bias caused by pareidolia.

Bias is disproportionate weight in favor of or against an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group, or a belief.[1] In science and engineering, a bias is a systematic error. Statistical bias results from an unfair sampling of a population, or from an estimation process that does not give accurate results on average.


The word probably derives from Old Provençal into Old French biais, "sideways, askance, against the grain". Whence comes French biais, "a slant, a slope, an oblique".[2]

It seems to have entered English via the game of bowls, where it referred to balls made with a greater weight on one side. Which expanded to the figurative use, "a one-sided tendency of the mind", and, at first especially in law, "undue propensity or prejudice".[2]