As a general rule, a fruit tree’s production depends on the overall health of the tree, its environment, its fruiting habit, the variety, the rootstock, adequate pollination, and good cultural practices. If just one of these conditions is off, the annual yield can be reduced or nonexistent. Apples are notorious for this kind of behavior. You may increase your chances of a good yield by early thinning of heavy fruit.

From The Old Farmer’s Almanac.