A few years ago, when I’d just started graduate work, I was waiting with a grandmother finishing a questionnaire I’d handed her about her 19-year-old granddaughter who was in the testing room next door. They were participating in a study that used brain imaging to look at adolescent development. The older woman had a soft southern accent and a warm laugh.

“What’s funny?” I asked.

“Cellphone use,” she said, gesturing at a question about how much time her granddaughter spent on her device. “When is she not on it? And my younger granddaughter, she just turned 18 – I mean, I think she’d die if you took the phone away from her.”

I told her that’s what I studied – “well,” I said. “Mostly social media.”

“Social media!” She shook her head. “Sometimes I think that’s the worst thing that ever happened to us.”

In the years since, that sense of broad, far-reaching harm has worked its way from our collective consciousness into thousands of high-profile lawsuits currently making their way through US courts. This week, yet another begins in New Mexico, focused on user data and the Cambridge Analytica scandal. In October, teenagers in Los Angeles will argue that Meta, YouTube and Snap caused serious mental health issues.

The most high-profile suits have focused on adolescent mental health, child sexual exploitation and, to a lesser extent, data privacy.

But in the years I’ve been researching how digital media is changing us, I’ve been increasingly concerned with how little we know about its cognitive effects, particularly those of short-form video, the format that now dominates every social media platform.

The vague sense that we are being cognitively hollowed out, dulled and diminished by our latest technology is captured in the term “brain rot”, recognized by the makers of the Oxford English Dictionary as the 2024 word of the year. And the more researchers look at it, the more it seems to describe distinct cognitive changes that are now being borne out by science.

The literature is still limited,........

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