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Why siblings are so different

If you have held your first child's milestones up against your second's and wondered how one home produced such different people, here is what the science, and my four daughters, taught me.

Same home, different children

I have four daughters: twins Pau and Balbina, born in 2016, Sofía, born in 2018, and Lucía, born in 2023. Same house, same parents, and they have never once developed on the same timeline.

Part of the explanation is temperament, the inborn style of reacting to the world. Alexander Thomas and Stella Chess's New York Longitudinal Study described dimensions such as activity level, adaptability, sensitivity, mood and persistence, visible early and fairly stable over time. Jerome Kagan showed that some babies react more strongly to anything new, detectable at around four months. The same parenting lands differently on different children.

Experiences also split apart under one roof. Our twins had nearly three years of onsite day care before the pandemic; Sofía grew up with a much smaller circle of other children.

Even twins: what Pau and Balbina showed me

We never learned for certain whether the twins are identical or fraternal, but very early they were obviously two different people. One has always been more sensitive to heat, cold, noise and spicy food. While my wife was breastfeeding, she once ate a Mangogo, a dried mango slice coated in chili. Pau didn't react at all. Balbina cried hard.

Their first steps came weeks apart, and while Balbina was still holding on to the furniture, she was the one talking and singing more clearly. She wasn't behind. She was on her own schedule. As I write in Chapter 2, "You cannot fight nature. You can mold it a little bit."

My younger child is slower than her sibling was. Is she behind?

Usually, no. The World Health Organization's motor study found that healthy children begin walking alone anywhere from eight to eighteen months, and language varies even more (see The range of normal is enormous). So watch the sequence, not the date: if your toddler isn't saying much yet, is she pointing, understanding words, communicating with gestures, eye contact and sounds? Then she is moving along the path at her own speed.

The pang you feel when you compare is human. It isn't a warning that something is wrong; it's a sign your children are becoming distinct people. The Ready for Now tool helps you switch from "Is she behind?" to "What is she ready for now?"

When is a difference worth a call to the pediatrician?

Variation is normal; a persistent absence is worth a conversation. If an expected skill hasn't appeared over several months, if your child loses something she could already do, if she doesn't turn to her name or look for your face, or if a worry keeps nagging at you, raise it with your pediatrician. Look for patterns over time, not one missed milestone. Asking early is not overreacting.

The full why

Why children from the same family, even the same womb, need such different support is the heart of Chapter 2 of Big Bang Baby. You can read the Introduction free.