When I had a second baby earlier this year, my three-year-old suddenly seemed enormous. "Check out the size of those feet!" I marveled. She seemed so heavy, so tall, so substantial.
She even seemed more capable, more robust. Images of airborne cookware and toppling bookshelves faded. The staircase didn't seem quite so treacherous. Instead, I trusted in her basic competence to scale the kitchen stools without incident and to keep (most) sharp-and-pointy things beyond the envelope of her person.
It wasn't just me: my husband reported the same experience.
I attributed these impressions to the contrast between a newborn and a toddler. The former was a meager handful of pounds that needed to be delicately scooped from one location to another. The latter was an energetic, 30-pound bundle of energy bounding toward me down the hallway. Of course the little one would seem smaller after spending time with the big one, and the big one bigger after spending time with the baby.
So when my colleague Lucia Jacobs, a comparative psychologist teaching a seminar on "the science of cute," sent me a recent paper on parental misperceptions of child size, I fully expected to have my "size contrast" theory confirmed.
I was wrong.
The paper, published last December in the journal Current Biology and authored by Jordy Kaufman and colleagues, did confirm that my experience is a common one. In a survey of 747 mothers with at least two children of different ages under the age of 6, the researchers found that over 70 percent reported that their erstwhile-youngest seemed bigger after the birth of a new infant.
A subsequent study suggested that contrast in size wasn't the primary reason why. Here's what researchers did in that study: they had 77 mothers with at least one child aged 1-6 estimate the height of that child by marking a line on a blank wall. The researchers then measured the child themselves (or had parents send in the child's height) to compute a "size estimation error" for different groups of children.
If the size-contrast theory is right, you'd expect the size of elder children to be overestimated, the size of younger children to be underestimated and the size of only children to be just right. But that's not what the researchers found. Instead, the mothers were most accurate when it came to their elder children, overestimating their size by an average of only 0.4 centimeters. When it came to youngest children — whether they had older siblings or were only children — the errors were more substantial: mothers underestimated the size of their youngest by an average of 7.5 centimeters.
What would generate this particular pattern of errors? And how does underestimating the height of one's youngest explain how my 3-year-old came to seem so big?
The researchers propose that mothers (and perhaps other caregivers) are susceptible to a "baby illusion" that makes them underestimate the size of their youngest child. When a new infant occupies the role of "baby," mothers cease to underestimate the height of their (now) second youngest, leading to the sense that this child has grown. In fact, the perceived height of this child has simply become more accurate.
To explain the origins of the baby illusion, the researchers turn to an evolutionary idea introduced by the ethologist Konrad Lorenz: that of baby schema, a suite of cute and attractive infantile features that elicit parental care and protection. If babies can effectively trigger the schema in their caregivers, then those caregivers are likely to help things along by perceiving infants in terms of the schema, which includes being small. As a result, infants are more likely to receive the care they need and parents are more likely to distinguish between their children in a way that supports a more appropriate allocation of resources based on each child's needs.
The researchers also suggest that the phenomenon could extend to characteristics beyond perceived size, such as perceived age or perceived cuteness. Perhaps it could even explain the shift in "perceived competence" that my husband and I experienced when our second baby came along: when the 3-year-old stopped being the youngest, we stopped underestimating her abilities.
I asked Jacobs — the colleague who pointed me to the study — what she made of the baby illusion hypothesis. She noted that the evolutionary explanation for the illusion still needs to be tested directly by measuring caregiving behaviors. That is, we don't know whether a misperception in size actually results in more appropriate care for infants or elder children. But she also noted that other studies do show that baby schema stimuli change basic cognitive and brain processes, focusing attention and increasing carefulness on abstract tasks.
I also queried another colleague, Frank Sulloway, the author of Born to Rebel: Birth Order, Family Dynamics, and Creative Lives. He was skeptical that mothers need a "baby illusion" to successfully differentiate their offspring and he offered an alternative evolutionary explanation for the findings:
The actual physical differences between older and younger siblings ought to provide sufficient cues for mothers to allocate resources in an appropriate manner, based on such considerations as the greater need of the younger sibling. For this reason, one is inclined to suspect that the function of a "baby illusion" is to adjust for some interfamilial process that would be maladaptive if allowed free reign, and that is not fully captured by the mother's perception of size differences.
My best guess would be that the illusion reflects the presence of parent-offspring conflict (as well as sibling conflict, which is the other side of the parent-offspring coin). Compared with the younger child, the older one is usually in a better position to influence the allocation of resources in his or her favor — for example, by pestering the parent, by feigning exaggerated need, and by acting aggressively toward the younger sibling . . .
To the extent that mothers are compelled to "give in" to such parent-offspring conflict, a baby illusion would help them to counteract this influence by altering the parent's perception of what the younger child needs relative to an older child — in essence, providing the younger child with a handicap. In short, these findings appear to reflect a process of weaning conflict, as set forth in Robert Trivers' classic 1974 paper on parent-offspring conflict.
Sulloway's alternative view is related to Kaufman's, but predicts that mothers with only children shouldn't fall prey to the illusion (at least, not to the extent that mothers with multiple offspring do), since they don't experience the conflict in resource allocation that Sulloway describes. Contrary to this prediction, however, Kaufman and colleagues did find that mothers underestimated the heights of only children by 5.6 centimeters (which differed significantly from zero), and that these underestimates weren't significantly different from the 9.9-centimeter underestimation that they found for younger children.
But here's the trouble: with a small sample size (77 children total, of whom only 17 were younger children and 17 were only children), one can't draw strong conclusions from the absence of a statistically significant difference between the height estimates for younger children versus only children. And if height underestimation is greater for younger children than it is for only children, then the "baby illusion" hypothesis can't be the whole story: an evolutionary alternative like Sulloway's — or the simple idea of size contrast — might play an important role.
Without more data, we can't do much to differentiate these competing proposals. But the phenomenon of having an older child seem to grow bigger when a new baby comes along appears to be a real one, as does the tendency to underestimate the size of one's youngest child.
Of course, one can always speculate. And I'd happily do so myself. But my tiny baby needs a diaper change and her enormous older sister's not quite big enough to do it on her own.
You can keep up with more of what Tania Lombrozo is thinking on Twitter: @TaniaLombrozo