No credible evidence that people are 'born lazy'

Wednesday April 10 2013

“Couch potatoes can't help being lazy – they were born that way,” claims the Mail Online website. 

As this is such a sweeping statement, it may be a surprise to readers that the science behind this headline was based entirely on rats and involved no human participants or gyms.

Researchers bred two distinct groups consisting of ‘lazy rats’ (rats that showed little to no interest in running in a wheel) and ‘active rats’ (rats that appeared to be highly motivated to run).

At the end of the 10-generation breeding programme, a series of tests was run to see if there were significant genetic differences between the two groups.

The researchers did find a number of genetic differences. However, in the main, the results were mostly inconclusive and did not shed much light on the potential biological causes for the differences in rats, let alone humans.

Even if the results were more ‘earth shattering’, an obvious limitation is that humans are vastly different to rats. Reasons for someone choosing to exercise is unlikely to be purely down to their genes.

So the immediate implications to humans are minimal. The headline is speculative and not supported by the research in question.

Where did the story come from?

The study was carried out by researchers from the University of Missouri (US) and was funded by a grant from the College of Veterinary Medicine at the University of Missouri and funds from the College of Veterinary Medicine’s Development Office.

The study was published in the peer-reviewed American Journal of Physiology.

The Mail’s reporting of this study was largely speculative and the implications to humans overstated. This was a relatively inconclusive rat-based study that was presented in the headlines as a relatively conclusive study of immediate relevance to humans. This isn’t the case in reality.

While the research is of some value – it at least proves that it is possible to selectively breed ‘lazy rats’ – it was not conclusive and its immediate practical relevance to humans is minimal. 

The headlines claiming “Couch potatoes can't help being lazy – they were born that way” and “genes play major role in deciding whether we enjoy a trip to the gym or not” are not backed up by the science behind it.

What kind of research was this?

This was an animal-based study examining the characteristics of rats that had been selectively bred to show high and low levels of voluntary running behaviour.

Rats with vastly different voluntary running behaviour were used to mimic the human condition whereby increasingly large swathes of the population are voluntarily inactive, while some remain very active.

As the study involved running voluntarily, the researchers hoped it may give clues to the origins of motivation to exercise.

Rat studies are often used, as the short lifespan of a rat means researchers can selectively breed a characteristic of interest (e.g. high voluntary running activity) in a relatively short space of time.

This allows researchers to mimic equivalent human evolutionary pressures, such as the shift from most people being physically active for much of the day to a more sedentary lifestyle. The equivalent study in humans would take decades, or possibly even hundreds of years.

Both rats and humans are mammals, so findings in rats usually give a sense of what might be happening in humans and forms the basis of further theories and explanations. But there is no guarantee that what is found in rats will be found in humans and this is why studies on humans are important.

What did the research involve?

The researchers started with 159 rats. When they were adults (28 days old) they were introduced to running wheels and the distance they ran voluntarily was monitored for six days.

After this period the 26 rats (13 males and 13 females) with the highest voluntary average running distances were separated from the rest and allowed to mate. This was repeated for 10 generations and subsequently selecting the top 26 voluntary runners in each generation.

Similarly, at the other end of the spectrum the 26 lowest voluntary runners were also selectively bred in the same way for 10 generations.

This ultimately led to two distinct, selectively bred groups of rats – ‘active rats’ and ‘lazy rats’

At the end of this process the researchers analysed aspects of the active rats and compared them with the lazy rats in an attempt to uncover what lay behind the differences in voluntary running characteristics. Factors that were studied included:

  • muscle characteristics in the hind limbs (the main muscles the rats use for running)
  • body fat and muscle composition
  • the way genes were switched on and off (gene expression) in the nucleus accumbens: a part of the brain thought to be associated with reward, motivating activities (e.g. running), as well as addictive behaviour such as drug addiction
  • gene expression in the muscles

The main analysis compared the characteristics between the active and lazy groups.

What were the basic results?

After 10 generations of breeding, the voluntary running distances (measured as an average distance on days five and six of a six-day running window) were 8.5 times greater in male active rats than male lazy rats (9.3km vs 1.1km, p<0.001). The difference in female rats was 11.0 times greater (15.4km vs 1.4km, p<0.001).

The active rats also ran faster and for significantly longer for both sexes.

The researchers thought physical inactivity might be a result of larger body weights causing the rats to exercise less. However, they actually found running patterns were not related to differences in body weight.

No differences were found for the amount of food eaten, body fat percentage or weight gained between the two groups. This may appear slightly odd as one might expect the runners to eat more to balance the energy expenditure of running, or to be thinner if they didn’t eat more.

No significant differences in hind limb muscle characteristics were observed between the groups.

The analysis of gene expression in the brain uncovered eight gene transcripts that were expressed differently between the groups (that is, having a greater than a 1.5-fold difference).

The top differences were related to genes the researchers described as involved in “cell morphology, cell death and survival, dermatological diseases and conditions” as well as “nervous system development and function, cell signalling, and molecular transport”. They did not go into further detail.

How did the researchers interpret the results?

The researchers concluded that their selectively bred rats “can potentially be used to further study low motivation for voluntary running and any other phenotype [characteristic] co-selected along with this trait”.

The researchers discussed the possibility that certain brain signalling pathways may explain some of the reasons behind differences in voluntary exercise, but these were largely speculative.

They highlighted their finding that “increased fat mass alone was not a factor driving lower voluntary running distances”, as previous research had suggested a causal link between having more fat and being less inclined to exercise. This was in addition to the stronger relationship in the other direction, that is, less active people have more fat as a result.


This small study provides future animal researchers with a unique and interesting group of rats to study genetic factors behind differences in levels of voluntary exercise. Through selective breeding, the researchers produced a group of rats that were highly motivated to run and another group that were not. The current study did not shed much light on the potential biological causes for the differences in voluntary exercise behaviour, but did provide a solid base for their study in the future – in rats at least.

The findings in these selectively bred rats have limited immediate relevance to humans. However, further research into the genetic basis of motivation to exercise based on this preliminary work may potentially lead to implications for humans, although this is likely to be a long way off.

The research findings themselves are very limited in telling us any reasons for the differences in the high and low running groups. However, they did observe a small selection of genetic differences that could provide a rough clue as to what was different in the two groups.

These genetic differences require a lot more research to confirm if they are indeed involved or important in exercise motivation in rats. Yet more studies would then be needed to see if similar genetic factors were present or important in humans. There is no guarantee that the differences found in rats will be found in humans – humans have to be studied directly to be sure.

The reasons why someone chooses to exercise or not is unlikely to be entirely down to their genetic make-up. It is likely that there are a wide range of underlying factors including cultural and psychological, as well as individual circumstances.

While this research may of be of interest to animal behaviourists and the like, its immediate implications to humans are minimal and were overstated by the media.

Analysis by Bazian
Edited by NHS Choices