adoptionMothers perceive their youngest children as shorter than they actually are, a new study has suggested.

This "baby illusion" applies regardless of the number of children a mother has, according to a team of Australian researchers.

In a survey of 747 mothers, they underestimated the height of their youngest child by an average of 7.5cm (3in) yet accurately judged the height of any older children they had.The study authors believe this is an adaptive mechanism - to nurture and protect most vulnerable offspring.

Lead researcher Jordy Kaufman, of Swinburne University of Technology, said: "Our research potentially explains why the 'baby of the family' never outgrows that label. To the parents, the baby of the family may always be 'the baby'."

The researchers found that more than two-thirds of interviewees remembered experiencing a sudden shift in their youngest son or daughter's size immediately after the birth of a new baby.

This perceptual shift primarily relates to the former "baby" of the family - mothers were less likely to report any height difference in other siblings.

This is not just because the older child looks so big compared with a baby, the researchers say. It actually happens because all along the parents were under an illusion their child was smaller than he or she really was. When the new baby is born, the spell is broken and parents now see their older child as he or she really is, they say.

The researchers asked 70 mothers to estimate - by putting a mark on a wall - the height of each of their children. The mothers consistently underestimated the height of their only or youngest children (aged two to six). Yet many were good at estimating the height of their older children and everyday objects, such as the bathroom sink or kitchen counter.