Why is it that we are more shocked by what happens to dead babies than to live ones? The story that almost 800 dead babies were buried in a disused sewage tank outside Tuam in rural Ireland turns out to be problematic. It is certain that 796 babies did die under the care of nuns in a home for unmarried mothers there between 1925 and 1961 and that is in itself a shocking statistic. But what gave the story wings was the claim that their bodies had been dumped in a septic tank, widely attributed to Catherine Corless, the local historian who uncovered the scandal.

In an interview she has denied that she ever used the term "dumped". More to the point, it was impossible that 800 children were placed there, since "only" 204 died in the years before the home was connected to the mains water supply, in 1937. In her first account of the discovery, Corless described the structure as a "crypt". Only later did she identify it, from a map, as a septic tank. If the bodies were placed in it long after it had been drained and disused, this would seem much less shocking. That less shocking story is at least plausible: the alternative would be that the nuns buried some babies decently in the unofficial graveyard but just dumped others in the cesspit. On what basis could they possibly have chosen?

This is not to deny that almost 800 children died and were buried in an unofficial graveyard behind the home. In the manner of these things in rural Ireland, it was both known and not known – according to the Irish Times "this small grassy space has been attended for decades by local people, who have planted roses and other flowers there, and put up a grotto in one corner".

Twenty babies dropped in a cesspit as corpses is a horrifying figure. Even one would be dreadful. And of course the whole story fits wonderfully into the larger stories of Irish nuns as heartless and cruel, which many undoubtedly were. But what's interesting to a student of religion is why the desecration of dead bodies should be so very much more shocking than the deaths of living babies.

Corless has established that about 20 children a year died in the home for the years of its operation, which could hold around 200 mothers at a time. That would make an infant mortality rate that is shocking by modern civilised standards though actually no worse than that of the whole of Ireland in 1910. But outside the Tuam home, it had dropped from 11% in 1910 to 3.7% by the end of the 50s. This progress does not seem to have reached into the home. That is rightly horrifying.

But it still doesn't horrify us in the same way as the thought of dead babies tossed into a cesspit does. Two explanations occur as possible. They may not be mutually exclusive: that's to say that they might be different ways of describing the same phenomenon.

The first is that we have an innate sense of the sacredness of dead bodies. That seems to be a factual and true claim. Certainly, the burial of the dead is one of the things that distinguishes humans from our ancestors, and one of the things that is held by archaeologists to distinguish skeletons of people like us from those of people who have not quite got to full humanity. That's why we think Neanderthals were humans, for instance.

The second is that we feel an instinctive sympathy for the figure of a woman holding a dead baby in a way that we don't when the baby is merely ill or suffering. That's a cruel thing to say, but again, I think it is actually true. Either way, we have here a fact about human nature that is terribly difficult to justify rationally. It takes a very cold heart to say that a dead baby is not worth our grief because it has passed beyond suffering.

This story will undoubtedly be used to attack religion. But what it actually shows is how very deeply religious instincts operate within us.