System 2 has a natural pace. Having random thoughts and monitoring what happens around you is not effortful. We make small decisions when we ride our bicycle, take in some information as we watch the news and have low key conversations with our colleagues or partner. These actions take little effort and can be compared to a stroll. It is usually easy to be walking and thinking at the same time, but in some cases they cause a mental overload. When you go on a walk with someone and you ask that person to instantly solve the problem 32 x 64, he or she will stop walking. Walking faster than your natural pace worsens your thinking ability, as your attention shifts to maintaining a faster pace. If you walk as fast as you can, it will be impossible to focus on anything else. Next to the psychical effort, it takes mental effort to fight the urge to slow down: self-control. Conscious thoughts and self-control fight over the same restricted budget of effort.

Sometimes people are in a state of effortless concentration in which the maintenance of a coherent train of thoughts requires no willpower. Psychologist Csikszentmihalyi called this a ‘flow’. Examples are car speeding, painting, playing chess, writing. Being in a flow state can make you lose your sense of yourself and time. Activities that induce this flow are called ‘optimal experiences’. These activities take considerable effort, but in a state of flow, the maintenance of focused attention on them requires no discipline. 'Flow' separates the two forms of effort: the deliberate control of attention (self-control) and concentration on the task (cognitive effort).

Psychological research has demonstrated that someone who is simultaneously challenged by a temptation and by a demanding mental task is more likely to give into the temptation. When you get the task to remember a list of numbers for several minutes and at the same time have to choose what you want to eat: broccoli or pizza, you are more likely to go for the pizza. System 1 has more influence on our behavior when System 2 is occupied. Someone who is cognitively busy is also more likely to use sexist language, be superficially judgmental in social settings and make selfish decisions. A busy System 2 loses the hold on behavior, although mental load is not the only cause of depleted self-control. Other possible causes are a bad night of sleep, drinking alcohol or anxiety about the task. Conclusion: self-control requires effort and attention. 

Experiments conducted by psychologist Baumeister displayed that voluntary physical, emotional and cognitive effort all – partly – drain our tank of mental energy. His experiments involved successive tasks. Efforts of self-control or will are tiring: if we have had to force ourselves to do a task, we are less likely to exert self-control when starting the next task. This is called ‘ego depletion’. Participants who had to suppress their emotional response did not do well in a later psychical test. Emotional effort has a bad influence on your ability to endure muscle pains. An ego-depleted person is therefore more likely to give up faster. In another experiment, participants who started with the task to eat healthy food while resisting sweet treats later gave up quicker than usual when faced with a demanding mental task.

Many tasks and situations lead to depletion of self-control. They all involve the need to suppress a natural urge and conflict. Examples are avoiding the thought of red cats, trying to impress someone and responding friendly to your husband’s bad behavior. There are also many and various indications of depletion, for example reacting aggressively to someone provoking you or not doing well in cognitive tasks.

Highly demanding tasks require self-control, while the exertion of self-control is unpleasant and depleting. Unlike mental load, ego depletion is a loss of motivation. It does not equal being cognitively busy.

Baumeister also found that mental energy is not merely a metaphor. The nervous system is one of the most glucose consuming parts of the body, especially when you are carrying out demanding mental tasks. Carrying out a cognitive activity that requires self-control results into a lower blood glucose level. This effect of ego depletion can be reversed by ingesting glucose. Only the participants who got a glucose drink before starting the second task were not depleted. Intuitive mistakes are usually more frequent among ego-depleted individuals.

A recent study showed the effects of depletion on judgment. Judges had to review parole applications. The researchers found an increase in approved requests after every food break. In the period until the next break, the rate dropped to nearly zero just before their next eating moment. The best explication is that hungry and fatigued judges had the urge to go for the easier default decision: denial of parole.

Monitoring and controlling actions and thoughts suggested by System 1 is one of the most important functions of System 2. System 2 either allows, suppresses or modifies them. Read the following puzzle, what does your intuition tell you? “An ice cream and chocolate dip cost € 1.10. The ice cream costs one euro more than the dip. What is the price of the dip?” You automatically answer € 0.10, which is wrong. If the price of the dip is € 0.10, then the total price will be € 1.20 (0.10 for the dip and 1.10 for the ice cream). The right answer is € 0.05. Answering with € 0.10 means that you did not actively review your intuitive answer and your System 2 supported a wrong answer that it could have prevented with little effort. Here we see the ‘law of least effort’ at work. Several thousand students at leading universities in the United States answered the puzzle and more than half of them gave the wrong answer. At lower ranked universities, 80% of the participating students failed to give the right answer. The ice cream – dip puzzle demonstrates that most people are overconfident: they are prone to put too much trust in their intuitions and avoid cognitive effort.

Now read the following argument and decide as fast as you can if the conclusion drawn is correct. “All apples are fruits. Some fruits are pink. Therefore some apples are pink.” Most college students agreed with the conclusion, but it is actually an invalid syllogism: it is possible that there are no pink apples. Because a plausible answer comes to mind straight away, not many people are willing to put effort into thinking it through. This is a problematic finding, because it indicates that when people believe a conclusion is valid, they also tend to believe the arguments are valid. System 1 focuses firstly on the conclusion, the arguments follow later.

Read the following question and rapidly give your best estimation. “How many homicides occur in the state of Tennessee in 12 months?” This question challenges System 2. The trick is whether people will remember that Memphis, a city with a very high crime rate, is in Tennessee. People from the United States know that Memphis is one of the biggest cities of Tennessee. The ones that remember that Memphis is in Tennessee give higher estimates. Most respondents who were asked this question did not think of the city when asked about the state and reported lower guesses than the respondents who were asked about the number of homicides in Memphis. Failing to think of Memphis can be a flaw of both System 1 and System 2. Whether the city pops up in your mind depends partly on the automatic function of memory. That is something people differ in. Some people, such as residents, have extensive knowledge about the state and are more likely to remember various facts about it. It also depends on the interests of people and their intelligence. Intelligence is not solely about reasoning, but also about retrieving relevant facts from memory and deploying attention. While memory function is associated with System 1, taking your time for a conscious search of memory is a feature of System 2. The extent of this search varies among people. 

The ice cream – dip puzzle, the apples syllogism and the Memphis – Tennessee question have one thing in common: giving the wrong answers seems to be caused by insufficient motivation, not making enough effort. Students of high-ranked universities have the capability to provide the right answer. Without the temptation of accepting a plausible answer that automatically comes to mind, they can solve much harder problems. It is troubling that they are so easily satisfied and stop thinking. Their System 2 proved to be lazy. They should be less willing to accept tempting answers, more alert and intellectually active and have less confidence in their intuitions.

Shane Frederick used his Cognitive Reflection Test to examine the characteristics of students who had performed poorly and found that they are prone to answer with the first thought that comes to mind and are reluctant to make the effort of checking this intuition. They are also prone to believe other ideas from System 1. The students were particularly impatient, impulsive and wanted instant gratification. 63% of them preferred getting a smaller amount of money now than a larger amount after a month. Only 37% of the students who had given the right answer to the puzzles went for the smallest amount. The findings of Frederick indicate that System 1 and System 2 have different ‘personalities’. System 1 is intuitive and impulse, System 2 is cautious and capable of reasoning, but can also be lazy. The same goes for people: some are like System 1, others like System 2.

The link between self-control and thinking was also examined by Walter Mischel. He subjected four year olds to a dilemma: receiving a small reward (one cookie) whenever they wanted it or a bigger reward (two Oreos) after waiting for 15 minutes in a non-distracting room. Half of them succeeded in waiting for 15 minutes, mostly by trying not to pay attention to the cookie. Over a decade later, the children that had managed to resist the temptation had greater measures of executive control in cognitive tasks, in particular the ability to reallocate attention. They were also less likely to do drugs and had better scores on intelligence tests.

Other researchers examined the connection between intelligence and cognitive control by exposing four to six year olds to computer games specifically designed to engage their attention and control abilities. The researchers discovered that training attention improved both executive control and their scores on intelligence tests. They also found that parenting techniques affected the kids’ ability of controlling attention and a close link between this ability and the ability of controlling emotions.