Survivorshop Bias

Survivorship bias is a cognitive bias that happens when we focus – sometimes unintentionally – only on the people or things that “made it through” a situation, and ignore those that didn’t. When that happens, we get a distorted picture of reality (1).

This bias shows up in many ways. Someone might say, “My great-uncle smoked his whole life and lived to 90, so smoking can’t be that bad.” But the people who smoked and died young aren’t here to tell their story – their stories are missing. The same thing happens when people say, “I didn’t get the vaccine and I was fine,” without considering the many who weren’t so lucky (1,2,3).

Survivorship bias can also affect research (1,3). For example, a study suggested that older adults who drank more wine lived longer than those who drank less or not at all – implying that drinking wine helps people live longer (4). But this can be misleading. Many people who were harmed by heavy drinking may have already died before the study started, so they weren’t counted. Others might have quit drinking because of health problems and ended up in the “non-drinker” group, making the drinkers look healthier by comparison (3,4,5).

Another great example comes from how the American military decided where to reinforce WWII airplanes. If they had only considered the bullet holes on the planes that returned, they would have missed a crucial piece of insight – those areas weren’t fatal. The real areas that needed reinforcement, were the places where bullets hit and caused planes not to return (6).

When we only look at people who completed a treatment or succeeded in a program, we miss those who dropped out or didn’t make it. That can lead us to believe something works better than it actually does (1,7,8,9).

This matters because it can lead us to make decisions based on incomplete data (1). Looking at both successes and setbacks helps us see the full picture – and make better choices.