Final answer:
Leah's observation illustrates the law of diminishing marginal utility, which implies that satisfaction decreases with increased consumption of the same item. It also touches on how portion size and food presentation can psychologically influence how much we eat.
Step-by-step explanation:
Leah's observation that her dinner guests ate less from a very large plate of cookies that were cut in half rather than whole best illustrates the concept known as the law of diminishing marginal utility. This economic principle suggests that as a person consumes more of a particular item, the satisfaction gained from consuming additional quantities decreases. This phenomenon can also be related to portion control and the psychological impact of food presentation. For instance, using a smaller plate can make the same-sized serving appear larger, thus leading consumers to eat less without feeling deprived. Additionally, when people have a large variety to choose from, and serving sizes appear smaller (such as halved cookies), they may perceive that they are consuming more, further influencing them to eat less overall.