The idea of parallel play idea dates back to Parten (1932), who noted the prevalence of this kind of behavior in young preschoolers. For fun, here's the key figure from her study:
So we turn to the internet. Of course, whattoexpect.com has an interpretation of why parallel play occurs:
[Parallel play is] par for the developmental course for babies and toddlers. Why? Because a child this age is still busy figuring out so much about the world and doesn't yet realize that people his own size are indeed people (who might actually be fun to do stuff with). He's too young to make friends, but companionable side-by-side play is a good start.You hear this echoed across many other sources of information for parents, including the teachers at M's daycare. These sorts of stage labels are endemic in developmental psych of the popular variety, and they often imply that there is a cognitive change that accompanies the behavioral stage shift. I think this developmental story is deeply wrong.
Over the last 15 - 20 years, a large body of evidence has accumulated that suggests that young children have very robust expectations for the social world by their second year. Babies can build social expectations for almost anything – even for eyeless blobs – so they definitely should have such expectations for other toddlers. Other work suggests that very small cues like reaching, looking, and movement towards a target can effectively cue inferences about an agent's goals and desires. So toddlers almost certainly understand that their peers have goals and desires, perhaps desires that even differ from the toddler's own. In addition, toddlers have no trouble engaging in reciprocal interactions with older children and adults (e.g., giving games, simple games of catch).
In fact, in a recent paper by Cortes and Dweck, having adults engage in parallel play – rather than reciprocal play – with toddlers made them less likely to help that adult achieve a goal later on. So that's a nice piece of evidence for two things. First, parallel play is far from being the only way that toddlers can interact. Second, they actually think it's negative in some way when an adult doesn't play with them reciprocally, so they are forming strong expectations both about and from the type of play they engage in with different partners.
Why do toddlers exhibit so little parallel play, then? I think what's going wrong is that the appropriate social cognitive abilities are very much present in kids of this age, but they are hard to exercise, and critically, social computations are slow. Reciprocal interaction with a peer requires fast online recognition of goals and action planning with respect to those goals. You need to know what your play partner wants you to do, and you need to figure that out before she loses interest and gets distracted. That's pretty easy for adults to do; they create structured play opportunities for toddlers all the time. (For example, last night I set up a tea party for M and helped her serve tea to a wide variety of different stuffed animals).
But when you get two toddlers together, they strike out so often that it might be adaptive to avoid trying to engage! In a recent episode I watched, M saw that another little girl Y wanted a toy car. But by the time she figured out that Y wanted the car, Y had already moved on to other things. The result was that M walked up to Y at a totally inappropriate time and thrust a car in her face for seemingly no reason. Nice idea, but poor execution. Maybe if you are a toddler, you learn not to try out this kind of gambit until you're more confident you will succeed.
This explanation – that parallel play is an adaptive consequence of toddlers' poor speed of processing – is a product of something that I've been exploring a lot on this blog: that babies and toddlers are surprisingly knowledgeable about the world, but their ability to use this knowledge is sharply limited. The limitation here is that social computations are very slow, so that by the time the computations are done, their output is less likely to be relevant. In other words, "parallel play" as a description is correct, but the shift to a more reciprocal style of play may not have anything to do with a cognitive shift. Instead it may emerge from more gradual changes in children's speed of social processing.
Cortes Barragan R, & Dweck CS (2014). Rethinking natural altruism: Simple reciprocal interactions trigger children's benevolence. Proceedings of the National Academy of Sciences of the United States of America, 111 (48), 17071-4 PMID: 25404334