Divided attention is a term used to describe the mental state where people try to focus on multiple things. Over the years, scientists have been very interested in the human ability to multitask in this way, and several studies have been performed to test the limitations and mechanics of attention splitting. Generally speaking, experts have found that people often pay a major price in performance when trying to focus on multiple tasks or information sources, and that the limits of divided attention are generally rather strict. As a result, some people feel that multitasking should generally be discouraged while performing certain tasks, but others feel that society should embrace it because it may offer certain advantages over more focused thinking in the modern digital age.
Any time people try to focus on more than one thing at a time, their brains have to divide their attention up between each task. For example, if someone tries to read an email while listening to someone talk at the same time, or attempts to watch a television show while simultaneously surfing the Internet on a tablet, that person is exhibiting divided attention. Some people have more of a tendency towards multitasking than others, and some experts feel that the modern availability of digital media and new forms of entertainment may be leading to a society where multitasking is much more common or necessary than it once was.
Overall, research has generally shown that people have a very strict limit on how many things they can pay attention to at once, and any amount of divided attention almost always leads to some level of performance degradation. This is generally thought to be the reason that distractions like cell phones have a tendency to cause more car accidents, and it may also contribute to poor performance in many other areas of life. There may be some ability to improve a person's capacity for multitasking through practice, but some experts think it may not be possible to avoid suffering a pretty significant decrease in effectiveness, even for very low levels of multitasking.
There are a lot of arguments on whether divided attention is always a good or bad thing. Most agree that sometimes it is better to be centrally focused on a single stream of data, but some experts think there are certain situations where multitasking may allow people to perform or learn better, while others think it is almost always best to avoid it if possible. For example, some college professors have actually prohibited their students from doing anything in the classroom at all, including a prohibition on note taking, because of concern that any distraction might limit the person's ability to learn. Others think that people learn in a new and different way when they are multitasking, and may benefit in ways that people haven't yet recognized.