Slackers may have brains that are wired for under-achievement, a study suggests.
Scientists have identified neural pathways that appear to influence an individual's willingness to work hard to earn money.
Scans showed differences between "go-getters" and "slackers" in three specific areas of the brain.
People prepared to work hard for rewards had more of the nerve-signalling chemical dopamine in two brain regions called the striatum and ventromedial prefrontal cortex.
Both are known to play an important role in behaviour-changing reward sensations and motivation.
But "slackers", who were less willing to work hard for reward, had higher dopamine levels in the anterior insula. This is a brain region involved in emotion and risk perception.
Dopamine is a "neurotransmitter" that helps nerves "talk" to each other by sending chemical signals across connection points called synapses.
Psychologist Michael Treadway, from Vanderbilt University in Nashville, US, who co-led the research, said: "Past studies in rats have shown that dopamine is crucial for reward motivation. But this study provides new information about how dopamine determines individual differences in the behaviour of human reward-seekers."
The findings are reported in the latest issue of the Journal of Neurosciences.
A total of 25 healthy volunteers took part in the research ranging in age from 18 to 29.
To determine their willingness to work for a monetary reward, participants were asked to perform a variety of button-pushing tasks.
An easy task earned one dollar (60p) while the reward for harder tasks ranged up to four dollars (£2.50).
After making their selection, volunteers were told they had a high, medium or low probability of getting a reward. Tasks lasted for about 30 seconds and participants had to perform them repeatedly for about 20 minutes.
Positron emission tomography (PET) brain scans, which monitor radioactive "tracer" molecules in the brain, were used to measure dopamine activity.
For many, the term "slacker" is personified by Jeff Bridges' character "the Dude" in the 1998 film The Big Lebowski.
The new research suggests that Lebowski's desire to remain unemployed and spend much of his time bowling could partly be down to brain chemistry.
Study leader Professor David Zald, also from Vanderbilt University, said: "At this point, we don't have any data proving that this 20-minute snippet of behaviour corresponds to an individual's long-term achievement, but if it does measure a trait variable such as an individual's willingness to expend effort to obtain long-term goals, it will be extremely valuable."
The scientists say they were surprised by the role of dopamine in the anterior insula. It suggests that more dopamine in this brain region is associated with a reduced desire to work, even when it means earning less money. In the past, dopamine has always been linked to reward-driven behaviour.
The fact that dopamine has opposite effects in different brain regions complicates the picture for treatments of conditions such as attention-deficit disorder, depression and schizophrenia.
Prof Zald's team is engaged in a larger project aimed at identifying objective measures for depression and other de-motivating psychological disorders.
"Right now our diagnoses for these disorders is often fuzzy and based on subjective self-report of symptoms," he said.
"Imagine how valuable it would be if we had an objective test that could tell whether a patient was suffering from a deficit or abnormality in an underlying neural system. With objective measures we could treat the underlying conditions instead of the symptoms."