Research has shown that anxiety disrupts people's ability to notice the unexpected.
In one experiment, people were asked to watch a moving dot in the centre of a computer screen.
Without warning, large dots would occasionally be flashed at the edges of the screen.
Nearly all participants noticed these large dots.
The experiment was then repeated with a second group of people, who were offered a large financial reward for accurately watching the centre dot, creating more anxiety.
They became focused on the centre dot and more than a third of them missed the large dots when they appeared on the screen.
The harder they looked, the less they saw.