In his fine recent biography of Constantine , Paul Stephenson explains the growth of Christianity as a result of “sex, health, and arithmetic.” The sex part has partly to do with abortion and infanticide, but more deeply with the basic family structure of ancient Rome: “If the population of the Roman Empire was sixty million at the time of Constantine’s birth, only around twenty-four million of these were women. Given that boys are more problematic in the womb, more sickly and more inclined to die at a young age in military activity or by violence, this figure is quite remarkable and can be explained only by the fact that baby girls were frequently murdered.”

He goes on: “It was rare for all but the wealthiest families to raise more than one daughter, however many were born . . .

“and infanticide was the surest way to dispose of unwanted girls. It was legal, philosophically justified and widely practiced. An infamous letter sent by a man to his pregnant wife in 1 BC instructs her: ‘if it’s a boy keep it, if it’s a girl throw it away.’ . . . far fewer girls than boys were allowed to grow to maturity, that is to child-bearing age, and consequently the general rate of reproduction in the late Roman world was kept artificially low.”

Abortion was widely practiced too, even though the procedure was dangerous and gruesome. The reason, Stephenson says, “was men. “Roman husbands and fathers had complete control over their wives’ and daughters’ fates, and could order that abortion be carried out as surely as they could order the exposure of a healthy baby girl.”

Christianity challenged all of these. Christians were forbidden to have abortions or to expose infants. Further, Christian women tended to marry later than pagan women and Christian men were expected to have sex with their wives, and only with their wives: “Christians discouraged marriage below a certain age and banned consummation of a marriage between a man and a child bride, such that the average age of marriage for Christian women became twenty, whereas for pagan women it was twelve. One must add that the rate of reproduction among pagans was very low: men favoured birth control (including anal and, less commonly, oral sex), indulged ion homosexual sex, took concubines and patronized both male and female prostitutes, who in turn favoured various methods of birth control and abortion when necessary. All of these practices were forbidden to Christians, as most were to Jews. Roman men who converted to Christianity were obliged to have vaginal intercourse with their wives, and if pregnancy resulted, were obliged to have a child and raise it, regardless of sex.” The church provided protection for women whose husbands attempted to force them to violate these standards: “A Christian woman would have a community to support any resistance she offered to the directives of a pagan husband to do otherwise.” The church thus grew more rapidly than pagans, and in particularly “there were rapidly far more Christian women than pagan women, as a proportion among their communities.”