We live contextually and comparatively. Something is high if it is above us; big if it is larger. A person seems successful if they are doing better than you are; fat if they are bigger than you are.
The New England Journal of Medicine reports that people become obese in social clusters: having a close friend or sibling become fat makes it more likely that you will too. The study does not conclusively show what is the mechanism behind this collective enlargement it but one likely candidate is readjusted norms. If your friends become larger, your sense of what is normal will adjust to accommodate them.
The New York Times reports that even if by most measures they are very wealthy, people feel poor if they are surrounded by people much richer than they are. As the article points out, while their lifestyle (80 hour work weeks, piles of debt) may say "struggling", most of these people do have the option of leaving: their wealth means they could move to many other places and live quite easily. To the extent that they have a problem, they have the means to solve it. More interesting is the other side - how much of feeling rich has to do with having more than others.
If what we really care about is our relative wealth, then wouldn't society be best off with minimal distinctions between levels - we'd be better at noting subtleties - the same people would be the richest and the poorest, but the whole range would be closer to the middle? Robert H. Frank addresses these questions in some new books and in the paper "Are Positional Externalities Different?"
Some questions to think about: if obesity spreads through social networks as norms change, how is this related to the issue of anorexia and media images of super thin models? Is the social network connection more about shared lifestyles or perceptions of body-weight - or both? Basically, the key question is to understand the extent and circumstances in which people's perceptions of "normal" or "ideal" are relative to others and when are they absolute - usually a mix of both (the poorest rich can still enjoy their luxuries, the richest person in a refugee camp can still be extremely uncomfortable)
