“The Second Demographic Transition: A concise overview of its development,” by Lesthaeghe, summarizes this concept:The SDT starts in the 1960s with a series of multifaceted revolutions. First, there was the contraceptive revolution, with the introduction of hormonal contraception and far more efficient IUDs; second, there was the sexual revolution, with declining ages at first sexual intercourse; and third, there was the gender revolution, questioning the sole breadwinner household model and the gendered division of labor that accompanied it.