Growth phenomenon 'baby illusion'

When a mother tells her first-born: "My, how you've grown", it could be due to a "baby illusion", scientists believe.

A perceived growth spurt in elder children after the birth of a new baby in the family could all be in the mind, scientists say

When a mother tells her first-born: "My, how you've grown", it could be due to a "baby illusion", scientists believe.

The birth of a second son or daughter often coincides with an apparent growth spurt in a parent's first child.

But this only because of the mind playing tricks and shrinking the size of the youngest child, whatever age he or she may be, say psychologists.

When the new sibling arrives the first-born child ceases to be the youngest, and therefore seems to shoot up in size overnight.

"Contrary to what many may think, this isn't happening just because the older child just looks so big compared to a baby," said Dr Jordy Kaufman, from Swinburne University of Technology in Australia.

"It actually happens because all along the parents were under an illusion that their first 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."

Dr Kaufman's team began by asking 747 mothers if they remembered experiencing a sudden change in their first child's size after giving birth for the second time.

In 70% of cases, the mothers said they had encountered the phenomenon. Their "erstwhile youngest" child suddenly appeared bigger after the new infant's arrival.

The researchers asked the mothers to estimate the height of one of their young children, aged two to six, by placing a mark on a blank wall.

They then compared the height of the marks to each child's real height. The results, reported in the journal Current Biology, showed that mothers significantly underestimated the height of their youngest child by 7.5 centimetres on average.

In contrast, height estimates for the eldest child were almost accurate.

"The key implication is that we may treat our youngest children as if they are actually younger than they really are," said Dr Kaufman.

"In other words, 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 findings are a reminder of how illusory perceptions of the world can be.

"We cannot trust the accuracy of our perceptions," Dr Kaufman added.

"In this case, it shows that our feelings and knowledge of our children affect how we actually perceive them. But it's important to consider that this miss-perception may actually make it easier to quickly distinguish one's youngest child from the other children."

The scientists wrote: "The baby illusion should ... contribute to greater affection and attentive care-giving to one's most needful child, regardless of that child's age.

"This is especially important in a one-child family where passing on one's genetic material depends on that child's survival.

"In the multi-child family, human and non-human animal research shows that parents experience conflict regarding how to divide their care and attention among offspring - providing more care to either the older or younger depending on a range of ecological and biological factors."

They pointed out that while the study focused on height, for some parents the "baby illusion" might take the form of a shift in perceived age or "cuteness".

The researchers added: "Our findings likely underestimate the degree to which parents experience some form of the 'baby illusion'.

"Regardless, we now know that mothers of new babies who were surprised and alarmed at the almost magical overnight growth of their older child are in good company."