Children in life may be larger than they appear

A few years ago, my then boss/PI and I submitted a paper to the journal Current Biology. It was rejected because they thought our paper was too specialized and not high impact enough. We were eventually published in another journal that had a higher impact factor than Current Biology (self-righteous science five!).


I share all this not just to humblebrag but to note that my feelings about Current Biology are colored by that rejection. You have been warned.

The paper: Kaufman, J., Tarasuik, J.C., Dafner, L., Russell, J., Marshall, S., & Meyer, D. (2013). Parental misperception of youngest child size. Current Biology, 23(24), R1085-R1086.

The abstract summary (the article is super short, just over a page, so I guess it gets a summary instead of an abstract): “After the birth of a second child many parents report that their first child appears to grow suddenly and substantially larger. Why is this? One possibility is that this is simply a contrast effect that stems from comparing the older sibling to the new baby: “everything looks big compared to a newborn”. But, such reports could be the result of a far more interesting biopsychological phenomenon. More specifically, we hypothesized that human parents are subject to a kind of ‘baby illusion’ under which they routinely misperceive their youngest child as smaller than he/she really is, regardless of the child’s age. Then, when a new baby is born, this illusion ceases and the parent sees, for the first time, the erstwhile youngest at its true size. By this account the apparent growth results from the mismatch of the parent’s now accurate perception with the stored memories of earlier misperceptions. Here we report that the baby illusion is a real and commonly occurring effect that recasts our understanding of how infantile features motivate parental caregiving [1].”

Hold up, y’all. Let’s see that first sentence again. “After the birth of a second child[,] many parents report that their first child appears to grow suddenly and substantially larger.”

This is a thing? I have never heard of this sudden and substantial increase in firstborn child size being a thing. I also googled the quote they gave in the summary, “everything looks big compared to a newborn”, and only got hits for this paper or pieces about this paper, so I am not convinced that “everyone knows newborns make things look giant” is a thing either. Maybe it’s a thing in the UK or Canada, where the authors are from?

These phenomena that the paper seems to refer to as common knowledge may actually be based on a survey from the paper itself: “Over 70% of respondents indicated that the erstwhile-youngest child suddenly appeared bigger after the new infant’s birth.”

Survey data aren’t much to hang your hat on, especially if they ask weird questions like, “Following the birth of your youngest child, did your other child suddenly appear bigger?” BUT the experiment based on the survey results has some pretty compelling data. When empirically tested, parents do think their youngest children are physically smaller than they actually are, while not-youngest children are pretty accurately sized.

Figure 1 from the paper

Figure 1 from the paper

The study design was elegantly simple (and annoyingly so; why are my studies so complicated?): they had parents draw on a wall how tall they thought their kids were. The experimenters then compared children’s actual heights to those estimated by their parents. Lo and behold (see above), parents estimated the heights of kids who were an elder sibling, meaning that they were not the youngest child, pretty closely to those kids’ real heights.

Kids who were youngest siblings, on the other hand, were consistently estimated to be smaller than they actually were. Note that this group included only children; additional analyses showed that only children and youngest children with older siblings didn’t differ from each other.

Wha?? The authors call this a “baby illusion” and speculate that it may be evolutionarily beneficial for youngest children, who need the most parental attention, to be perceived as smaller and weaker than they actually are. Interesting speculation, though I am a bit dubious. Runts and weakest of the litters are usually ignored/left to die in the wild to save parental resources for those who are guaranteed to survive, so it may not always be evolutionarily adaptive to be perceived as weak. I wonder if the “baby illusion” doesn’t apply for twins, then?

I hereby propose the testing of a “puppy illusion”, a “kitten illusion”, and a “baby panda illusion“. Please leave a comment below if you will give me funding to do science with all the cute baby animals. I already have extensive experience watching Bao Bao on the National Zoo Panda Cam.

Screen Shot 2014-02-14 at 10.05.28 PM

Personal screen shot from Panda Cam

Yup, baby pandas definitely look tiny. I estimate that I can put her in my pocket and take her home with me.
Kaufman J, Tarasuik JC, Dafner L, Russell J, Marshall S, & Meyer D (2013). Parental misperception of youngest child size. Current biology : CB, 23 (24) PMID: 24355780


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