Studies have shown that no matter how much they have eaten during a previous meal, children tend to overeat if served large portions. This may be because there is a strong human tendency to finish a serving of food, no matter what the portion size.

Researchers at the University of Pennsylvania experimented with a bowl of M&Ms placed in the lobby of an apartment building with a serving spoon positioned next to it. Over the course of time, the spoon was replaced with ones of increasingly larger size.

The researchers found that no matter how small or large the spoon was, passersby tended to take one portion, or a spoonful of candy - and finish it.

This observational study, which was reported in a recent issue of Psychological Science, helps to explain why consumers are influenced by larger portion sizes such as super-sized fries or drinks. Rather than drinking or eating only a normal portion size, people feel compelled to finish the serving.

The message for parents? Learn what constitutes a normal serving size of the foods you give your child - and serve only that.