New US research has found that just having your smartphone nearby reduces cognitive capacity and the ability to complete tasks -- even when it's switched off.

Carried out by researchers from the McCombs School of Business at The University of Texas at Austin, the study set out to try to measure for the first time how well people can complete tasks when they have their smartphones nearby, but are not using them.

The team recruited nearly 800 smartphone users, who in the first part of the study were asked to sit at a computer and complete a series of tests that required full concentration in order to attain a good score.

The tests were designed to measure participants' available cognitive capacity -- the brain's ability to hold and process data at any given time.

Participants were randomly instructed to either place their smartphone either face down on the desk, in their pocket, in their bag, or in another room.

Regardless of where their phone was, all participants were instructed to turn their phones to silent.

The researchers found that participants who put their phones in another room significantly outperformed those with their phones on the desk, and slightly outperformed those participants who had kept their phones in a pocket or bag.

The findings suggested that just the presence of their smartphone reduces individuals' available cognitive capacity and reduced cognitive functioning -- even though people feel they're giving their full attention and focus to the task at hand.

"We see a linear trend that suggests that as the smartphone becomes more noticeable, participants' available cognitive capacity decreases," commented McCombs Assistant Professor Adrian Ward, "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. It's a brain drain."

In another experiment, participants were asked to report on their own "smartphone dependence," or how strongly a person feels he or she needs to have a smartphone in order to get through the day.

Participants then performed the same computer-based tests and were again randomly instructed where to keep their smartphone, although in this experiment some participants were also instructed to turn them off.

Results showed that participants who reported being the most dependent on their smartphones performed worse than those who were less dependent, but only when they kept their smartphones on the desk or in their pocket or bag.

The team also found that it didn't matter whether a person's smartphone was turned on or off, or whether it was lying face up or face down on a desk.

The researchers believe the reduced cognitive ability is because part of the participants' brain was actively working during the tests to not pick up or use the phone.

"It's not that participants were distracted because they were getting notifications on their phones," explained Ward, "The mere presence of their smartphone was enough to reduce their cognitive capacity."

The findings can be found published online in the Journal of the Association for Consumer Research.