secular trend

The secular trend is the slow, generation-over-generation shift in how children grow: over the past century or so, people in many countries have become taller and have entered puberty earlier than their grandparents did. It is the kind of change you can only see by comparing whole generations, not individuals.

Doorways in old houses, antique furniture, and historical army records all hint at it. Better nutrition, cleaner water, fewer childhood infections, and improved overall health let children realize more of their genetic growth potential, so average adult height rose markedly across the 1900s in many populations.

Alongside greater height, the average age of puberty has drifted earlier. The age at menarche in many countries fell substantially over the same period, and breast development now tends to begin somewhat earlier than in past generations, a change linked to nutrition and rising childhood body fat among other factors.

Two honest caveats matter. First, the trend reflects populations realizing their potential under better conditions, not a change in the underlying genes, and it tends to plateau once living conditions are good. Second, the causes of earlier puberty are still debated and likely multifactorial, so the secular trend is a robust observation but not a single tidy mechanism.