Experiments dating back to the 1960s show people have less of a reaction to viewing an unpleasant image or experiencing an electric shock when they know it’s coming than when they’re not expecting it. That’s because uncertainty, a long-known cause of anxiety, makes it difficult to prepare for events or to control them.
People vary in their desire to minimise uncertainty. Those who react by worrying focus on potential threats and risks such as “what if I don’t get the promotion?” or “what if I get sick?”. Worry can be useful when it leads to adaptive behaviours that reduce threat, but chronic worry may cause harmful levels of stress that can affect heart health and the functioning of the immune system, among other things.