Having a smartphone handy makes you dumber, new research has found, and it doesn't matter if it's turned on or off.
But there's a simple fix - put it away.
Researchers at the University of Texas conducted two experiments with around 800 smartphone users to see how phones affected people's cognitive ability.
In the first, participants took a series of difficult tests that required full concentration. Some had their phones sitting on the desk, others were asked to keep it in their pocket or bag, and the rest left their device in another room entirely.
The best scores came from those whose phones were out of the room. Bag phone participants were a distant second, and the worst performers were those with the temptation sitting right on their desk.
"Your conscious mind isn't thinking about your smartphone, but that process - the process of requiring yourself to not think about something - uses up some of your limited cognitive resources," said study co-author Prof Adrian Ward.
"It's a brain drain."
The second experiment found those who rely heavily on their phone just to make it through the day were affected more heavily, but only when the phone was in their bag or on their desk. Putting it in another room saw them perform just as well as those who don't need to be connected 24/7.
In both experiments, it didn't matter if the phone was off or on - the effect was the same, so the effect can't be blamed on distractions like notifications and phone calls.
"It's not that participants were distracted because they were getting notifications on their phones," said Prof Ward. "The mere presence of their smartphone was enough to reduce their cognitive capacity."
The study's results were published in the Journal of the Association for Consumer Research.