Many of us believe we can do two things at once. We try it every day even though our limitations are obvious.

Yet we persist, so much so that laws need to be passed to deal with our foolishness. For instance, it is illegal in all Australian states and territories to drive a motor vehicle and use a mobile phone at the same time, without some sort of hands-free arrangement.

Such laws stem from the recognition that driving safely requires considerable attention. Operating a mobile phone also requires attention, which takes away from the more important task of driving.

In some states, police have begun fining pedestrians who use mobile phones while crossing the road.

Young people often claim they are experts at multitasking. That they can monitor several electronic devices at once makes it seem to be the case.

But research regularly demonstrates when they try to do two things at once, they tend to do both tasks poorly. Either they make more errors or they take longer than they would if they did one thing at a time.

We’re limited to how much attention we can devote to any one task. Mike Kegley/Flickr, CC BY

In one study, university students' laptops were monitored by a spyware program during lectures. It found students attempted to multitask by checking course material and taking lecture notes as well as looking at emails, engaging in social media, surfing the web and playing games.

The study found the more often students engaged with non-course-related material during the lecture, the worse their academic performance was in the course.

Some researchers claim females do show an advantage in multitasking, but most have failed to find any significant gender difference.

Three main principles explain well our limitations in performing multiple tasks at the same time.

1. Some tasks require a lot more attention than others

Walking, chewing and the physical act of talking seem to require so little attention that we can do them without even thinking.

By contrast, constructing an argument, reading a book and following a movie all require considerable amounts of attention – particularly if we want to do the task well.

2. We are limited in how much attention we can devote to any task or tasks

This seems to be a limitation our brain is built with. If doing two tasks together requires less than maximum attention capacity, then we are likely to pull if off.

So, for instance, most people would find it trivial to walk along a path and have a discussion with a friend.

By contrast, if two tasks together exceeded our attention capacity, then something would have to give.

So even though driving and having a conversation might seem fairly easy to most people, if the road conditions suddenly changed and the act of driving became more challenging, then the conversation would likely stop.

Our maximum attention capacity is also affected by our arousal state. If we are tired, for instance, we just don’t seem to have the same attention capacity as when we are fully awake.

3. We can get better at multitasking

Our ability to perform some tasks can improve with practice. This often means the particular tasks come to need less attention, even to the point of becoming automatic.

In a recent study of mine, participants were presented with pictures of randomly arranged dots and asked to count them.

The time it took them to respond was directly related to the number of dots in a picture: the more dots, the slower the response. But after seeing every picture many times, their responses were no longer related to the number of dots.

In fact, participants were equally fast regardless of whether there were six or 11 dots. They knew the answer automatically rather than having to consciously work it out through a counting process.

A similar process underlies our acquisition of many cognitive skills, such as reading words.

Novice drivers usually struggle to hear basic instructions while they are driving because all their attention is devoted to keeping the car moving smoothly and avoiding other vehicles.

But after a few years of driving experience, this task requires far fewer cognitive resources. Some are then freed up to perform other tasks, such as singing along to the radio or thinking about the best route home.

The important thing to note is that not all tasks can be practised to the point where they require little attention to perform. Such tasks, by their very nature, always require most of our attention.

Holding a serious conversation with someone is not something we can shunt off to automatic pilot and expect a worthwhile outcome.

So, can we do two things at once? It depends on the nature of the tasks we want to perform simultaneously, how aroused we are, the extent of our experience with each of the tasks, and how much we care about the quality of our performance.