The results showed that those who kept their phone on their desk (in eyesight) scored 10 percent lower on questions that tested focus and memory. They also reacted slower to speed tests. In fact, even when their phones were turned off or on “do not disturb” mode, if they were on the desk, the participant scored lower than those whose phones were in the next room.
This means smartphones diminish a person's working memory capacity and "fluid intelligence" – the ability to solve novel problems independent of already stored information, which is called “crystallized intelligence”.
Unsurprisingly, they found that the negative effects of having your phone nearby was greater for those who self-identified as being dependent on their phones. They think this effect is not because the smartphone user’s mind is being distracted by thoughts of checking for messages, but rather their mind is concentrating on trying not to be distracted, at the expense of the performance of the task at hand.
"Your conscious mind isn't thinking about your smartphone, but that process – the process of requiring yourself not to think about something – uses up some of your limited cognitive resources,” concluded lead author Dr Adrian Ward in a statement. “It's a brain drain."