How Babies Remember: A Tribute to Carolyn Rovee-Collier

Because babies can’t speak to tell us what they know, and because they don’t know how to encode whatever they have learned into language when they begin to talk, all kinds of peculiar claims have been made about their knowledge and memory of their experiences. these range from the idea that babies remember their prenatal lives right back to conception, to the belief that they remember being born and can articulate these memories later, to the assumption that they remember nothing at all even at age 2  years “because they’re too young to notice”.  It’s been pretty rare for people to figure out ways to investigate how babies learn and how their memories work.

One of the few people to create a method for looking at infant memory was Carolyn Rovee-Collier, whose death in 2014 was a great loss to the field of developmental psychology. Rovee-Collier’s work was an excellent example of something I’ve always thought to be true—that in order  to have real insights into ways to study babies, one has to have spent a whole lot of time with them. (No doubt what I am about to say will offend some readers, but I think this is a bit like what Daniel Lehrman and other comparative psychologists used to say about the study of animal behavior.) There’s a sort of total immersion that occurs when you are paying most of your attention to a baby for a long period of time. You begin to see things that were invisible to you before, just as an experienced dancer sees things in a dance performance that even the most enthusiastic non-dancer will miss.

Not all parents observe carefully, but Carolyn Rovee-Collier did while one of her children was an infant. She noticed that when her baby had discovered that by kicking his feet he could make a mobile swing, he kicked again when put back into the same setting, even though things had changed so he could not make the mobile move. At a few months of age, he had learned and remembered something he could do to make an interesting event happen.

Later on, Rovee-Collier systematically investigated the circumstances of learning and forgetting for infants of different ages. Her method—originally used to entertain her baby so Mom could get some work done!—was formalized under the name “mobile conjugate reinforcement paradigm”. The baby is put in a crib with a colorful print crib liner, a mobile is provided, and a ribbon is tied to the baby’s ankle and to the mobile so kicking will make the mobile move. (Do I need to say that babies are not left alone with the ribbon tied? This could be quite dangerous, so don’t try it at home unless you are staying nearby!) Whether the baby learns is measured by the increase in kicking; whether he or she forgets is measured by noting how long it takes for kicking to resume when the baby sees the print crib liner again. The baby learns not just that an action causes interesting results, but that the action works in certain circumstances and not in others, just like the “time, manner, place” learning that makes up so much of what we try to teach small children.

Just as you might expect if you didn’t think babies could remember their own births, younger babies learn more slowly and forget more quickly than older ones. Two-month-olds take 7 to 9 minutes to learn to make the mobile move, and forget  how it works in two days. Three-month-olds learn in 4 to 6 minutes and remember for 6 or 7 days. Six-month-olds learn the trick in 1 to 3 minutes and remember for 15 or 16 days, and after about 9 months the memory lasts for many weeks.

Now, here is the interesting thing that shows how the mobile conjugate reinforcement paradigm is relevant to ordinary real life learning. Babies remember longer when their memories are “reactivated” by letting them see (just see, not make the mobile move) either the mobile or the print crib liner at some time before they are tested. The timing of the reminder is critical, and it seems that some time must pass before the memory is affected by the repeated experience. For three-month-olds, at least 8 hours must pass before the memory is reactivated as shown by kicking in response to the crib and mobile, and it takes 3 days for the maximum effect of the reminder. A three-month-old who gets two memory reactivations (reminders) will remember what was learned for twice as long as the first learning lasted. By age 6 months, only an hour need pass before the reactivation effect is seen, and it takes only 4 hours to reach the peak effect.

The take-away message from Rovee-Collier’s work is that younger babies do learn, but they learn slowly and forget quickly unless they are reminded, not by having exactly the same experience as before, but by seeing the setting in which they learned. Even with a couple of reminders, three-month-olds remember the event for only two weeks. This suggests that a single experience in infancy is not likely to be remembered long, and that early learning that lasts will be likely to involve the many repetitions of familiar daily caregiving routines in familiar places. Only after about 9 months do we see quicker and longer-lasting learning. Another implication, and an important one, is that babies are motivated to learn not so much by gratification of physical or even emotional needs, but by the need to experience mastery over the environment and to make interesting things happen.  

It’s good to keep these things in mind when confronted by claims of prenatal and birth memories and their lifetime influences.