Many biases arise from the ways our memory and attention are built. Hindsight bias, often called the "I-knew-it-all-along" effect, makes past events feel predictable after they have occurred. Recency bias gives the most recent information disproportionate weight when making judgments, while the primacy effect makes first impressions especially sticky. The peak-end rule, demonstrated by Daniel Kahneman, holds that we evaluate experiences largely by their most intense moment (the peak) and how the experience ended (the end), rather than by the average of the whole. Negativity bias and its close relative, the negativity effect in impression formation, explain why negative events and negative traits leave deeper marks than equally intense positive ones; one rude act can outweigh many kind ones.
Attention itself is a limited resource, and biases arise from what it captures or misses. Inattentional blindness, famously illustrated by Simons and Chabris's invisible gorilla experiment, describes our failure to notice an unexpected but visible object when attention is engaged elsewhere. Change blindness is the related failure to detect changes in a scene, especially when the change occurs during a brief visual disruption such as a saccade or a cut in a video. Selective perception filters incoming information so that confirming data pass through while disconfirming data are blocked; fans of opposing sports teams can watch the same game and see different fouls. The halo effect extends these tendencies: a strong positive impression in one area, such as attractiveness, spills over into judgments about unrelated traits, such as intelligence or kindness.
Memory is similarly selective. The Zeigarnik effect explains why unfinished tasks linger in mind more than completed ones, which is why cliffhangers in television shows are so compelling. The bizarreness effect shows that unusual or odd material is more memorable than common material, a principle leveraged by mnemonic techniques that use vivid imagery. Pareidolia is the perceptual cousin of these patterns, as the brain insists on finding meaningful shapes, especially faces, in random stimuli like clouds, toast, or electrical outlets. Subjective validation reinforces similar beliefs: when we already hold an expectation, we perceive connections between unrelated events as evidence for it, which underlies belief in astrology, psychic readings, and many superstitions. The nocebo effect shows that expectations can even shape bodily experience, since negative expectations about a treatment can produce real side effects, even with a placebo.