Abstract
People often react to low probability, high damage events in which many die with strong avoidance behaviour. Indeed, analyses of behaviour following the September 11 terror attacks on New York City suggest that this caused a substantial number of additional, 'indirect' deaths as many people avoided flying for 12 months afterwards and took to the relatively risky highways of the US instead. We argue that such responses may have arisen as an adaptation to risks that wipe out a significant proportion of all carriers of an allele if they strike, e.g. storms. These are environmental fluctuations known as environmental or aggregate risks. At the opposite extreme, demographic risks affect individuals independently. We show that evolution by natural selection in fluctuating environments means it is adaptive to inflate environmental (aggregate) risks relative to demographic risks, where the inflation factor depends on the proportion of carriers of the allele that die if the risk strikes.