The prevalence of obesity worldwide has increased substantially in recent decades, particularly in the US (Ogden et al. 2002). The subsequent adverse consequences for youth health and medical care costs have led policymakers to search for innovative methods of preventing youth obesity.
One common proposal is to require that young people spend more time in physical education (PE) classes. For example, the US Surgeon General recommends that US school systems require students to participate in PE for at least 150 minutes per week (US DHHS, 2010). Many youth fall short of this recommendation; on average, elementary school students in the US spend less than 90 minutes per week in PE, according to data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-1999 (ECLS-K).
Implicit in the Surgeon General’s recommendation is the belief that increased time in PE lowers the probability of youth obesity. However, there are at least two reasons why this may not be true. First, even if young people spend more time in PE, they may not spend more time being physically active. A review of the literature found that less than half of PE time consists of moderate-to-vigorous physical activity (Pate et al. 2011). Second, children may engage in offsetting behaviour; that is, they may decrease physical activity outside of school to offset the increase in PE time.
Although there is a large amount of research on the correlation between time in PE and childhood obesity, few studies estimate the causal impact of PE on weight. Cawley et al. (2007) use variation across states and over time in PE unit credit requirements for high school students as instrumental variables for PE enrolment, and could not reject the null hypothesis of no causal effect of PE on the probability of obesity among high school students. Datar and Sturm (2004) find that the increase in time in PE between kindergarten and first grade decreases body mass index (BMI) only for overweight or obese girls.
In Cawley et al. (2012), we estimate the causal effect of PE time on physical activity and obesity by instrumenting for PE time using state requirements about the minutes of PE per week for elementary school students. We examine ECLS-K data for 1998 through 2004, merged with state PE requirements. The ECLS-K data contain measured height and weight, teacher reports of the amount of time that young people spend in PE, and parental reports of various types of physical activity. We find that mandates establishing a minimum amount of time in PE raise the amount of time children are in PE class, although there is less than full compliance. For example, in the 2004 ECLS-K data (which corresponds to the 5th grade), an additional ten minutes of PE required by the state increases the actual amount of time in PE by four minutes.
Our IV models indicate that additional time in PE lowers a child's BMI z-score and the probability of becoming obese; these results are robust to a variety of specifications and to a series of falsification tests. Increasing the amount of time in PE up to the recommended amount of 150 minutes per week would lower BMI z-scores by 8% of a standard deviation, which is 12% of the mean, and would reduce the probability of obesity by four percentage points. Looking at gender we find that the reduction in BMI and obesity occurs only for boys. We find evidence suggesting that the lack of an impact of PE on weight for girls is due to offsetting behaviour – when they spend more time in PE they become less physically active outside of PE. We find little evidence of such offsetting behaviour for boys.
We also examine the effect of state PE requirements on academic achievement. As states increase the amount of time in which students must participate in PE classes, a potential unintended consequence is that academic classes may be crowded out, resulting in lower test scores. Alternatively, PE may facilitate student learning and achievement by improving student health. Our results suggest that PE time does not affect achievement test scores. Rather than crowding out academic courses, increased PE is accommodated by extending the school day or reduced time in music education.
The results of this study represent some of the first evidence that physical education for elementary schoolchildren has a causal impact on obesity. Thus, it confirms a critical assumption behind recommendations by the US Surgeon General and others that PE time should be increased in order to reduce the risk of childhood obesity.
Cawley, John, David Frisvold, and Chad Meyerhoefer (2012), “The Impact of Physical Education on Obesity among Elementary School Children”, NBER Working Paper No. 18341.
Cawley, John, Chad Meyerhoefer, and David Newhouse (2007), “The Impact of State Physical Education Requirements on Youth Physical Activity and Overweight”, Health Economics, 16(12):1287-1301.
Datar A and R Sturm (2004), “Physical education in elementary school and body mass index: evidence from the early childhood longitudinal study”, American Journal of Public Health, 94(9):1501-1506.
Ogden, C L, K M Flegal, M D Carroll, and C L Johnson (2002), “Prevalence and Trends in Overweight Among US Children and Adolescents, 1999-2000.” Journal of the American Medical Association 288(14), 1728-1732.
Pate, Russell R, Michael G Davis, Thomas N Robinson, Elaine J Stone, Thomas L. McKenzie, and Judith C Young (2006). “Promoting Physical Activity in Children and Youth: A Leadership Role for Schools: A Scientific Statement from the American Heart Association Council on Nutrition, Physical Activity, and Metabolism (Physical Activity Committee) in Collaboration with the Councils on Cardiovascular Disease in the Young and Cardiovascular Nursing”, Circulation, 114(11): 1214-1224.
US Department of Health and Human Services (2010), The Surgeon General’s Vision for a Healthy and Fit Nation, US Department of Health and Human Services, Office of the Surgeon General, January.