Researchers claimed that parents routinely misperceive their youngest child to be smaller and younger than he or she really was.
Jordy Kaufman of Swinburne University of Technology in Australia, said that contrary to what many may think, this isn't happening just because the older child just looks so big compared to a baby.
It actually happens because all along the parents were under an illusion that their first child was smaller than he or she really was and when the new baby is born, the spell is broken and parents see their older child as he or she really is, Kaufman said.
Kaufman and his colleagues made the discovery first by asking 747 mothers if they remembered experiencing a sudden shift in their first child's size after the birth of a new infant.
The researchers found that 70 percent of the mothers did.
To further explore that perceptual shift, the researchers asked mothers to estimate the height of one of their young children, aged 2 to 6, by marking a blank wall.
When the researchers compared those height estimations to the child's real height, they found something very interesting: mothers significantly underestimated the height of their youngest child by 7.5 cm on average. In contrast, height estimates for the eldest child were almost accurate.
Kaufman said that the key implication is that we may treat our youngest children as if they are actually younger than they really are and their 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', Kaufman added.
The study is published in the Cell Press journal Current Biology.