The researchers used the scans to compare the volume and thickness of the cerebral cortex, the part of the brain responsible, inter many alia, for things like sensory perception, language and cognition, with that of the sub-cortex, a disparate collection of structures such as the hippocampus (involved in long-term memory formation) and the amygdala (which regulates fear). They confirmed that there is a small but consistent decrease in the volume of the cortices of new fathers after the birth of their child.

This shrinkage is not, however, evenly distributed. The biggest reductions are in the area at the back of the cortex where information from the retina is processed and interpreted, and in the “default-mode” network, a piece of neural circuitry distributed between three different cortical areas, which is associated with daydreaming, mind-wandering and thinking about the self and others.

This pattern partly mimics changes found in the brains of first-time mothers. For example, a study published in Nature Neuroscience in 2017 by some of the same researchers found that areas of the default-mode networks of their brains also shrink.

The differences in first-time fathers’ brains are less pronounced than those in mothers, and also more variable—and are, presumably, caused in a different way. But different physiological means can still arrive at the same evolutionary end.

They found that postpartum changes in brain volume predicted both how attached a mother felt to her child and the absence or otherwise of hostility to it.

It showed that those with the largest reductions in brain volume had the strongest MRI responses to images of their own child compared with images of others.