Haig delved into clinical case reports on patients with four rare genetic disorders. He found evidence that children with disorders characterised by dominance of some maternal genes -- Silver-Russell syndrome, Prader-Willi syndrome, and Temple syndrome -- place fewer demands on their mothers' resources.

For example, newborns with all three disorders display a weak desire to nurse, and slower childhood growth in general. Many also show early onset of puberty, which often marks a point at which children become less dependent on their mothers' sustenance.

Conversely, babies with Beckwith-Wiedemann syndrome, in which some maternally derived genes are suppressed and paternal genes dominate, are born heavy with particularly large tongues.

These individuals usually end up being tall, owing to their rapid growth both in the womb and as young children. They have a high frequency of childhood cancers, said a Harvard release.

These findings were published this week in the Proceedings of the National Academy of Sciences.