A recent and growing literature considers the economics of weight change and obesity. The leading questions have been "what accounts for the observed rise in obesity over time?" [Chou et al 2004, Lakdawalla and Philipson 2002, Cutler et al 2003] and "why do people (especially rational agents) choose to be overweight?" Because body weight can be adjusted by diet and exercise, "obesity is an avoidable state" and "economists expect these adjustments in behavior to take place if the benefits of adjustment exceed the costs" [Philipson 2001, 1].

But many overweight people prefer not to be overweight, as the existence of a sizable diet industry suggests. Americans pay $40-$100 billion annually to help themselves lose weight. The Wall Street Journal recently reported that "at any time, 29 percent of men and 44 percent of women are on a diet" [Parker-Pope 2003, R-I]. Even if these estimates are high, it is hard to gainsay the fact that millions diet with the aim of losing weight.

This paper asks "why do people diet?" The proximate answer is "to lose weight." But because there are different ways by which a person becomes heavier than he wants to be, the ultimate causes of the decision to diet are different. This has theoretical and empirical implications that we explore with a simple graphical model that determines desired weight and shows how different causes induce dieting.1

The paper proceeds as follows. The first section discusses the physiology of weight determination. The next section sets out a plausible list of diet causes. The following section analyzes these alternative causes within a general production function/utility framework. A simple graphical exposition shows how an individual's optimal weight is determined, yielding propositions about "optimal overweightedness." There follows a section which shows how several of the causes of dieting identified earlier can be usefully analyzed using this graphical framework. A final section presents conclusions about implications, applications suggested by the analysis, and possible extensions.

BACKGROUND: THE PHYSIOLOGY OF WEIGHT DETERMINATION

The production function for weight determination begins with the view that weight gain results when energy (calorie) intake exceeds energy use. Calories are expended in exercise, digestion of food, and "basal metabolic rate" (BMR), the latter being the energy the body expends when at rest. Basal metabolism is in fact the largest source of energy expenditure. A standard result in the physiology/nutrition literature is that BMR declines with age.

That metabolism slows with age suggests the following proposition: if an individual maintains the same level of calorie intake and exercise as he or she ages, that person will gain weight. This happens because energy (calorie) intake is constant, but energy use declines. Indeed, in Suranovic-Goldfarb-Leonard 2002 [hereafter SGL 2002] and Suranovic and Goldfarb 2006 [hereafter SG 2006], we harness a widely used empirical relation from the physiology literature, the Harris-Benedict equations, to obtain numerical estimates of the decline of BMR - and therefore calorie expenditure with age.2 These estimates are then used to generate weight-change scenarios.

The proposition that weight will rise with age even with constant calorie intake is consistent with evidence that weight does in fact rise with age. Costa and Steckel [1997, 55] examine body mass index (BMI) by age (from age 19 to 72) for a number of cross-sections from 1864 through 1991.3 The 1991 cross-section, for example, shows body mass index rising from between 23 and 24 at age 18-19 to between 26 and 27 at age 50-64, then falling to a little below 26 at age È5-79.4 Cutler et al [2003] also find that weight increases with age up to an age between 50 and 55.5

These empirical findings about weight gain with age, and the underlying contribution of falling BMR, provide important information for our modeling of weight choice and the incentive to diet.

A TAXONOMY OF CAUSES OF DIETING

People diet in an attempt to lose weight, but there are varied causes of perceived overweightedness. Understanding possible motives for dieting seems an important step in deepening our ability to analyze dieting phenomena both theoretically and empirically. In this section we provide a provisional taxonomy of diet causes, including: (i) "aging-associated"' dieting; (ii) "disease-provoked" dieting; (iii) "physical-life-eventsprovoked'' dieting; (iv) "style-provoked" dieting; (v) "smoking-cessation" dieting; and (vi) "innovation-provoked" dieting. Brief elaborations follow.

(i) "aging-associated" dieting. This kind of dieting stems from the fact, discussed in the previous section, that weight increases with age, given constant calorie intake. The age-associated weight gain may create incentives to diet. This motive is investigated in both SGL 2002 and SG 2006.

(ii) "disease-provoked" dieting. This kind of dieting stems in the most extreme case from what might be called the "diet or die" motive. An individual is diagnosed with a medical condition requiring that he lose weight to reduce threats to health or even life.