Thursday marks the 50th anniversary of one of the historic mistakes of the Catholic church: the decision by Pope Paul VI to restate the traditional ban on contraception in his encyclical Humanae Vitae. The immediate result was a crisis of authority within the church which led to tens of millions of Catholics for the first time deliberately defying the church’s teaching, and tens of thousands of clergy encouraging them to do so in language that was more or less coded. In the long term the damage was far worse and more widespread. Over succeeding decades, the encyclical was used to justify anti-contraceptive policies in the developing world which must have led to the avoidable deaths of tens of thousands from HIV/Aids.

While Catholic women in rich countries can, and do, ignore the encyclical entirely, the church’s pressure on politicians in donor nations, and its position as a major supplier of healthcare in Africa, mean that women in the developing world have no such choice. The victims of this encyclical are not the westerners who so loudly and publicly rejected it. They are the women of the global south whom the church claims in other contexts to champion. There are nearly 100m unplanned pregnancies in the world every year, the overwhelming majority in places where women have no access to contraception. Around 56m of them end with abortions, something that in Catholic teaching is very close to murder, and that is a statistic for which Pope Paul VI must bear a large share of the blame.

Yet it can’t be said that the encyclical was malevolently intended. Read with charity, it is an argument to share decisions about women’s fertility between men and women, which is certainly an advance on seeing them as a matter entirely for men. But the distinction between natural and artificial birth control, so philosophically important to the encyclical, is in practice the difference between contraception that works almost all the time, and the sort that needs everything to go right. This is true even when the rhythm method is marketed as a means of women’s empowerment, to be used with a thermometer and a smartphone app. But an entire generation of Catholic women discovered this when they tried to practise the teaching religiously back in the 1960s, and their daughters and granddaughters still know it. That is the kind of knowledge that cannot be unlearned.

The distinction between effective and ineffective contraception did not seem to matter very much to the Catholic church at the time because it believed that the “meaning and purpose” of sex is pregnancy. Couples had no real right to decide over their own fertility. That was for God to do – and the church, which claimed to know what God wanted. The pope may have believed in mutual decision-making, but in the end his actions showed that he could not practice it. Women were not really to be trusted with their own fertility. The consequence of that arrogance has been immense, and avoidable, suffering.