The US psychologist Paul Fitts wrote a paper in 1954 called The information capacity of the human motor system in controlling the amplitude of movement (https://www.ncbi.nlm.nih.gov/pubmed/13174710) which was published in the Journal of Experimental Psychology. Fitts' work would go on to be one of the most well-studied models of human motion.

To dumb Fitts' Law down for us UX people, rather than psychologists, the core concept that applies to us is:

If you're building a user interface, it's really simple to do this: make buttons big enough, and close enough, that users can efficiently find them and move between them:

A great anti-pattern example is those tiny "x" buttons to close pop-up ads: it's almost as if the advertisers don't want you to close them…