“He’s going to be really tall!” exclaims your mother-in-law, watching your 2-year-old run in circles around the room.
“How would you know?” you reply, attempting to protect him from the glass table.
“Height at 2, multiplied by two. Everyone knows that! He’ll be well over 6 feet.”
The 2 x 2 rule, popular on the internet and at playgrounds, feels like a classic old wives’ tale. Of course, old wives are not always wrong. Is this actually a good way to predict your child’s future height?
As it turns out, child height is predictive of adult height, generally speaking, particularly after the age of 2. And although it’s crude, the 2 x 2 approach is not much worse than the best that scientists can offer.
Collecting the relevant data is a heavy lift, because predicting adult height from child height requires researchers to see people both as children and as adults. Most of the data come from relatively small longitudinal studies. For example: One of the oldest papers on height, published in 1946 by the psychologist Nancy Bayley, relied on data from her California clinic, and from the Harvard Growth Study of 1922 to 1935. Those data sets included a few hundred children each. Bayley’s successors have relied on comparably small sample sizes. Because most of the relevant studies have yielded similar results, however, there’s no reason to dismiss them.
To convert data into some kind of predictive formula, scientists have generally used skeletal age as one of their inputs. That is, rather than just relying on how old the child is, they also incorporate information from X-rays to mark skeletal growth. The more developed the skeleton, the less future growth is expected.
But doctors don’t measure skeletal growth unless they have a clinical concern. To the rescue comes “A Chart to Predict Adult Height from a Child’s Current Height.” The paper, by Tim Cole and Charlotte Wright, is as advertised: The researchers crunched longitudinal data to provide a simple predictive equation, and a chart.
The chart shows the correlation between child height at various ages and adult height. At birth, the correlation between birth height (or length) and adult height is about 0.4, meaning that only about 16 percent of the variation in adult height can be explained by knowing height at birth (16% = 0.4²). This is a fairly weak correlation.
By age 4, the correlation is about 0.8 for boys and 0.66 for girls. That is, for boys you can explain about 64 percent of the variation in adult height by knowing height at age 4. This is a reasonably strong correlation, and means that kids who are tall when they’re 4 will likely be tall as adults.
Because the correlation is not perfect, however, you shouldn’t necessarily expect a very tall 4-year-old to be very tall as an adult. Imagine, for example, that you have a 4-year-old boy who is 3 feet 9 inches tall. The average 4-year-old boy is about 3 feet 6 inches, so your son is taller than average, right around the 90th percentile. An adult in the 90th percentile is about 6 feet 1 inch, but there’s a good chance your son will end up closer to the 5-feet-9-inch average for adult men—he'll probably grow to about 6 feet even. *
Only risk takers, moreover, would want to bet on that number. To arrive at 95 percent confidence, you need to expand out from a precise prediction to a range—5 feet 10 inches to 6 feet 7 inches. That is, anywhere from just a touch taller than average to not out of place on a professional basketball court. Even at the most predictive point in childhood—age 4—there is still a lot of noise.
This fuzziness gets even worse in early adolescence, when the predictive power of child height really drops off. Kids enter and exit puberty at different times; and puberty changes how tall kids are in the short term, but doesn’t strongly affect height trajectories in the longer term. (For an adult-height-prediction calculator, click here.)
With this background, we can return to the 2 x 2 rule. The average height at age 2 is, in fact, about half of the average adult height, and the correlation between height at 2 and height in adulthood is about 0.75 for boys and 0.65 for girls—about what the chart would yield at that age. You can’t know for sure what height your toddler will reach in adulthood, but the old wives don’t seem to have been too far off.
* This article previously misstated the height of a male adult in the 90th percentile. It is about 6 feet 1 inch, not 6 feet 4 inches.