“DNA matters for social equality” is the central message of Kathryn Paige Harden’s new book ‘The Genetic Lottery’. In this adapted extract, she argues that we must escape the trap of assuming that acknowledging biological difference leads inevitably to the justification and compounding of social hierarchy.

My older child struggled to talk. When he was two, he had only a few words, and the pediatrician offered us reassurances—he was fine, just be patient, boys can be late talkers. Six years later, he’s been in hours and hours of speech therapy every week. Therapists have reached into his mouth to hold down the front of his tongue so he can say “cookie” and “go.” He practices holding his jaw and rounding his lips and saying the correct number of syllables before he draws another breath.

During his speech therapy appointments, I sit in the waiting room and read with my younger child, who was a precocious talker. Her speech development felt miraculous in comparison with her older brother’s. What had to be relentlessly practiced with one child emerged with seeming effortlessness in another.

Why can one of my children talk with ease, while the other one labors to be understood? No one can give me a definitive answer. But I can look to twin studies and see that speech problems are over 90 percent heritable. Most of the differences between children in their ability to articulate words are due to genetic differences between them. The genetic influences on speech problems also appear to influence motor skills more generally, a scientific finding that comports with my personal experience. I watched my late talker struggle to learn to crawl, to walk, to ride a scooter.