It’s a common observation that short parents often have taller children. This seems counterintuitive since height is largely determined by genetics. So why do genetically short parents frequently produce taller offspring? There are several factors that can lead to this outcome.
Regression Toward The Mean
Regression toward the mean is a statistical phenomenon whereby extreme values in a population tend to move back towards the average over time. In terms of height, two very short parents are likely to have genetic heights that fall on the extreme low end of the normal distribution curve. When these parents have children, the children’s height is expected to regress upwards towards the mean height.
This doesn’t mean the children will be average height. They will likely still be shorter than average due to their genetic inheritance. However, they will tend to be taller than their extremely short parents simply because their genes are regressing towards the population average.
Assortative Mating
Assortative mating refers to the tendency for people to choose mates with similar traits, including height. So very short people tend to partner with other very short people. This amplifies the genetic tendency towards short stature in their offspring.
When the offspring of these pairings grow up, they may not continue mating assortatively. A shorter than average person born to very short parents may end up choosing an average height partner. This would shift the height of the offspring towards the mean.
Epigenetics
Epigenetics involves genetic changes caused by environmental influences, rather than changes to the DNA sequence itself. There is evidence that malnutrition during childhood can suppress genetic height potential. For example, studies of populations that suffered severe childhood malnutrition show taller average heights in subsequent better-nourished generations.
This indicates nutrition and other environmental factors can affect the expression of height genes. If short parents grow up in impoverished conditions but their children do not, the children may have more of their genetic height potential realized.
Secular Trend in Height
Average heights around the world have increased significantly over the past century. This secular trend in height is driven by improvements in nutrition, healthcare, and standard of living. Shorter parents born a century ago probably did not reach their full genetic height potential due to worse conditions during development. If they have children today, their children’s height will benefit from the secular trend to be taller.
Late Growth Spurts
Some individuals, particularly males, undergo late adolescent growth spurts and continue growing into their early 20s. If both parents completed their growth as teenagers, their offspring may have a delayed growth period and ultimately surpass the parents’ heights. Although less likely, growth hormone abnormalities can also sometimes cause children to far surpass their parents’ heights.
Regression to the Mean in Action
To illustrate regression to the mean, let’s look at some sample height data for parents and children:
| Parents Height | Children’s Average Height |
|---|---|
| Mother: 5’0″, Father: 5’3″ | Son: 5’7″, Daughter: 5’5″ |
| Mother: 4’10”, Father: 4’11” | Son: 5’5″, Daughter: 5’2″ |
| Mother: 4’11”, Father: 5’1″ | Son: 5’9″, Daughter: 5’6″ |
In each case the children’s average height is taller than the parents’ heights. But the children are still short to average, not extremely tall. This demonstrates a regression upwards towards the mean, rather than an extreme jump upwards.
Other Considerations
While the factors described explain why short parents often have taller children, there are exceptions. Some children may inherit short stature genes from both parents and be similarly short or even shorter. Medical conditions that severely impair growth can also stunt children’s height regardless of their genetic potential. And some tall children are simply genetic outliers who strongly inherit tall genes from more distant ancestors.
Conclusion
In summary, children born to very short parents commonly surpass their parents in height. But they usually remain relatively short themselves, just taller than their extremely short parents. This phenomenon is largely explained by regression toward the mean for height in the population. Assortative mating, epigenetics, secular trends in growth, and late growth spurts also contribute. Together these mechanisms typically result in a modest generational increase in height for children of very short parents.