There are 80 to 90 billion neurons in your brain. If just a tenth of those were set aside for recalling things, you’d be able to store a solid one billion individual memories up there, according to fancy statistical models.
Yet we forget things all the time. (My favorite, apparently, is misplacing my glasses—the very things I see with!—at least once a day.) To University of Toronto researchers Blake A. Richards and Paul W. Frankland, this prompts a question: why, after 2.8 million years of human evolution, would we be so programmed for forgetfulness. In a new review paper in the journal Neuron, they argue that forgetting isn’t a bug—it’s a feature.
This comes down to two main things.
Forgetting allows you to be more mentally flexible.
If you were stuck remembering everything about everything, like some kind of King Midas of the mind, it would be harder to adapt to our unrelentingly changing world. Persistence of memory “is only useful when it maintains those aspects of experience that are either relatively stable and/or predictive of new experiences,” the authors write. Absence is presence: not knowing how to do something means you’re freer to learn how to do it, rather than simply going with what worked before.
Frankland co-authored a 2016 study on mice that showed how this worked. His team trained two groups of mice to navigate a maze to find a goal. After they grew accustomed to that task, half the mice got their brains zapped, prompting the growth of neurons, which, for reasons too complex to get into here, induces forgetting. Then the experimenters moved the goal and let the mice back in there. Lo and behold, the absent-minded mice found the platform more quickly—their learning was improved through forgetting.
Anthony Wagner, the principal investigator at the Stanford Memory Lab, gave a more human example to me for an earlier story. “If you didn’t forget, you’d have this sea of interference that would make it hard to figure out which bit of memory helps,” he says. Consider the all-too-familiar case of the email password. After months of familiarity, you know your current one exceptionally well. But then security software asks you to get a new one. As much as you’re learning the new password, you’re forgetting the obsolete one. Together, that process allows you to remember the newly dominant one, and be much less frustrated with life.
Forgetting helps you generalize.
In popular culture, memories are usually expressed in the method of Bruce Springsteen: gold-hewn recollections of glory days past. But the poet laureate of Asbury Park, New Jersey, doesn’t portray an evolutionarily aware understanding of recall. Daphna Shohamy, who runs a neuroscience of learning lab at Columbia, once emphasized to me that memory is more action-oriented than nostalgic. “Our brains are built to help us deal with the world in better ways and not just so we can reminisce,” she told me. “Predicting the future is remembering what happened in the past. Our memories are a bridge between what happened and what will happen next.”
In order to take a specific experience and make it generalizable, you need to abstract the most important bits out and let go of the details, in the same way that Picasso’s drawings of bulls got more and more abstract. Like in the below series, you can tell the last figure is of a bull, even though there’s only a few strokes of the pen—no longer a specific steer, what remains is now the general form of a bull.
Richards and Frankland contend that the same thing is happening with memory. You lose details to gain generalizability, and we experience this as forgetting. “Memories can be viewed as models of the past,” they say, as “simplified representations that capture the essence, but not necessarily the detail, of past events.” That way, when your “environments are noisy”—when life is messy, weird, and unpredictable—your pruned down memories can be applied to more situations. Weirdly enough, learning from your mistakes requires forgetting, at least in part, about them.