Bias is common in mental-processing tasks as diverse as target recognition, heuristic estimation and social judgment. This paper holds that cognitive biases stem from the covert operation of neural modules, which evolved to subserve adaptive behavior. Such modules can be innate or forged early in development. Research shows links between (i) biases in cognitive tasks and (ii) neural devices, which may mediate them. Evidence is included from biases that arise spontaneously in artificial neural networks during recognition/decision tasks. Two linked propositions follow. First, there are continuities in biasing strategies across different levels of cognitive processing. Second, a proclivity for stereotyping and prejudice depends on the biased functions of lower-level neural modules that promote adaptations to social environments. The propositions rest on evidence of biological preparedness for stereotyping and of deficits in social judgment in patients with neurological lesions. To test such claims, research studies are suggested at the boundary of cognitive neuroscience and social psychology. Advantages of bias and prejudice as evolved tools may include their: (1) speeding of scrutiny and improving of target detection in changing or uncertain situations; (2) aiding of a rapid choice of practical short-term rather than optimal longer term plans; (3) allowing appraisal of a workable world by creating fairly stable categories; (4) motivating of exploration and completion of problem-solving which might otherwise be abandoned too early. The biological priming of social biases need not mean that they are immutable; understanding them could lead to better ways of controlling them.