It is a common experience for many parents that a newborn’s arrival makes an older child suddenly seem huge. I experienced this illusion myself when our second baby arrived, and was amused, but not terribly surprised, to see our 3-year old’s overnight transformation from toddler to little boy. Almost two years later, when our daughter was born, the metamorphosis happened again in the blink of an eye. Our almost 2-year old baby boy was no longer a baby, but an older brother to our new baby girl... who now remains the family’s “baby” despite being more than two and a half years of age. No younger siblings in the horizon, in case you’re wondering.

Many scientists, myself included, had thought that the abrupt size increase for the no-longer-youngest child was a matter of contrast. That is, that a young child seemed small in size because everybody else in the family was bigger. But a newborn’s arrival served as a new frame of reference to now correctly perceive the formerly youngest kid as the correct size. The explanation makes intuitive sense, yet it’s wrong. Recent research published earlier this year in Current Biology indicates that human parents are subject to a previously unknown “baby illusion” that makes them misperceive their youngest child as smaller than he or she is, regardless of age.

Figure 1. Actual and estimated heights of Youngest- and Elder-children plotted as a function of child age. Youngest-child estimates were significantly lower than their actual heights while Eldest-child estimates did not differ significantly from their actual heights. Individual estimation points are denoted with ‘E’ (dotted lines) and individual actual measured heights are denoted with ‘A’ (solid lines).












The researchers, led by Jordy Kaufman of Swinburne University of Technology in Melbourne, Australia, asked mothers to estimate the heights of one of their children (aged 2–6 years) by making a mark on a wall. Their estimations were then compared to the actual heights of the children. The results showed that mothers underestimated the heights of their youngest children (which included only children) by an average 7.5 cm, whereas they estimated the heights of older children roughly accurately (overestimated by an average 0.4 cm).

The authors of the study hypothesize that the baby illusion, where the parents misperceive their youngest children as exaggeratedly small, may have an adaptive value, leading to greater parental care of, and allocation of resources to, the youngest child.

Future research will tell if the illusion also explains the stereotype (or is it an accurate perception?) of the youngest kid as spoiled rotten. As an oldest child myself, and several objective inches shorter than my kid sister, I can’t wait to find out.