The more people who witness an emergency, the less likely any one of them is to help.
In experiments from the 1960s onward, people who believed they were the only witness to someone in trouble helped quickly. When they believed others could also hear, most waited, assuming someone else would act.
Why is this true?
Responsibility gets divided among everyone present, and people look to each other for cues. First-aid training now teaches you to point at one person and say: you, call an ambulance.
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Psychology facts describe the shortcuts your mind takes without asking permission. We misjudge randomness, remember interrupted tasks better than finished ones, and change our answers when a group disagrees with us. The entries here stick to findings that have held up under replication, avoid the pop-science versions that overstate them, and explain what the effect really predicts about everyday behaviour.