Do tall parents always have tall children?

Statistically tall, yes. Predictably tall, no.

The Galton phenomenon - named after Francis Galton, who first measured it in the 1880s - is called "regression to the mean." Galton found that very tall parents had tall children, but their children tended to be slightly shorter than them on average. Very short parents had short children, but their children tended to be slightly taller than them on average. The next generation drifts back toward the population average.

The reason is statistical. Adult height is influenced by thousands of genetic variants. Tall parents tend to have many of the "tall" variants but not all of them. Their children inherit a random subset and on average regress toward the population mean.

The rule of thumb that paediatricians use: a child's adult height tends to fall within a range called the mid-parental height - average the two parents' heights (adjusted for sex), with a confidence interval of about 8–10 cm on either side. So two parents who are both 180 cm tall will most likely have children in the range of 170 to 190 cm, with the centre of the distribution slightly below 180.

Nutrition and childhood health also play significant roles. A genetically tall child who was malnourished in early childhood may not reach their genetic potential. A genetically average child with excellent nutrition may exceed expectations.

Tall parents tilt the odds. They do not guarantee outcomes.