Study: Why We Shrink With Age

PROBLEM: As gravity pulls down on us, as the cartilage between our joints wears down, and as our spines are weakened by osteoporosis, we shrink. The process, as with most processes associated with aging, is degenerative. But just as people age differently, people shrink differently -- and how many inches we lose may tell us something about how we've lived our lives.

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METHODOLOGY: Economists at USC, Harvard, and Peking University analyzed data from 17,708 Chinese adults over the age of 45. They estimated the former, full height of older participants who had already started to shrink by looking at the length of their arms and legs (limbs don't shrink), and then extrapolating from younger participants with similar limb length. They then compared participants' loss in height to their performance on an array of tests and health evaluations, along with their responses to survey questions.

RESULTS: With age, men tended to shrink an average of 3.3 centimeters, or 1.3 inches. Women lost 3.8 centimeters, or an inch and a half. The researchers discovered a number of health and lifestyle indicators of height loss. City people shrank less than country folk. Illiterate men and women lost .9 and .6 cm more, respectively, than men and women who completed primary school; completing high school appeared to prevent an extra centimeter from being lost.

Height loss, they also found, is also an indicator of other losses that come with age. It's most closely related to a decline in cognitive health. Poor performance on tests of short-term memory was associated with an increased loss in height; people who had lost more height were also less able to say what the date was or solve basic math problems.

IMPLICATIONS: Although it's partially determined by genetics, adult height is also influenced by factors established in childhood, like how much sleep we got and how healthfully we ate while we were still growing. At the extreme, poor health and nutrition can lead to stunted growth. We can look at the standing height of an adult population and make certain, reasonable assumptions about their childhoods. 

What the researchers show here is that height loss, too, can tell us something about how healthy we are. In adulthood, lifestyle factors that wear heavy on the bones, like drinking, smoking, and inactivity, promote shrinkage. Factors like education and where we live may affect our health -- and height -- in less obvious ways, perhaps because of the ways in which they are related to those lifestyle factors. And while it isn't clear, either, how a causal link between a decline in cognition and a decline in stature could possibly work, efforts to promote and preserve cognitive health might help older adults remain tall, or vice versa. 

The full study, "Health, Height, Height Shrinkage, and SES at Older Ages: Evidence from China," is published in the American Economic Journal: Applied Economics.