The changing social gradient in age at menarche across cohorts and generations in Norway

Age at menarche (AAM) is an important milestone in women's life-courses. A secular decline in AAM has raised concerns, as early AAM has been associated with adverse health and social outcomes. Still, it remains unclear whether the decline differs by socio-economic position (SEP) and drivers of such differences. Using data from 122,826 Norwegian women born 1960-2008 whose childhood SEP was assessed from registers, we document a declining trend and a growing positive social gradient in AAM. We further used a balanced panel of 10,896 mother-daughter dyads in the Norwegian Mother, Father and Child Cohort Study (MoBa) to decompose the inter-generational decline in AAM within each income quintile into within-family and between-family components. Mean AAM declined by 0.44 years in the lowest quintile compared to 0.24 years in the highest quintile, but there was no systematic difference between the quintiles in the within-family component. This suggests that factors operating through childhood conditions to causally affect AAM have similar impacts across SEPs. Rather, we find earlier AAM among mothers whose children grow up in a low SEP than for mothers who themselves had low SEP during childhood, which suggests that maternal selection is the main mechanism for the emerging gradient.