Screen Time and Teen Depression: What 21,797 Twins Showed

In Sweden, a twin study of 21,797 young people just put one of the loudest parenting fears of the decade through its hardest test yet. The paper, by a Karolinska Institutet team, was published on October 8 in Nature Human Behaviour, a peer reviewed journal, and it is open access.

What the full sample showed

Twins in the Child and Adolescent Twin Study in Sweden reported their screen habits at ages 9, 15 and 18 and filled in depression and anxiety questionnaires up to age 24. The answers were linked to national registers of diagnoses and antidepressant prescriptions, followed to about age 28.

Across everyone, the familiar pattern appeared. At 15 and 18, more than six hours a day of screens, compared with two hours or less, came with depression and anxiety scores up to 0.51 standard deviations higher in girls and up to 0.22 in boys. Heavy interactive use at 15 (gaming, chatting, browsing) on school days came with 3.37 times the risk of a later diagnosis or antidepressant. Heavy TV and video watching showed no clear link with diagnoses.

What happened inside twin pairs

Identical twins share their genes and their home, so comparing the twin who used screens more with the one who used them less strips out much of what families pass on. Inside identical pairs at 15, the symptom links shrank by 32 to 95% and none stayed significant after correction for multiple testing. The diagnosis risk fell to 1.65 times within twin pairs, a 57 to 59% reduction, and also lost significance after correction. Symptoms at 15 predicted slightly more screen use at 18 (odds ratio 1.04), so the arrow runs both ways.

The estimates did not reach zero. At 18, heavy weekend interactive use within identical pairs still came with symptoms 0.26 standard deviations higher, a size the authors compare with bullying. Per extra hour, the effect was about 0.03 to 0.05.

The verdict

Screen hours have been counted as if they were the cause, when they mostly travel with things that are harder to count: sleep, stress at home, temperament. Blanket two hour rules chase an average that this study barely moves. The open question sits at the extreme end, where six hours a day crowds out everything else, and that is where the next study should look.