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Nature vs Nurture - The Stimulating Environment


This was a landmark study by Mark Rosenzweig almost 60 years ago, with super important implications. The experiment has also been replicated many times over. 

Rosenzweig wanted to investigate the effects of the environment on brain growth and development (commonly known as neuroplasticity - read more about it here). He and his colleagues selected only male rats (to eliminate the variable of gender differences) from different litters (to ensure that results are not confined to just one particular lineage), and randomly allocated them to 2 distinctively different environments:


  • the impoverished environment: rats were placed in a small and empty cage, with nothing except food and water. The rats were also isolated, with no interactions with other rats.

  • the stimulating environment: this was like a mini Disneyland for rats. There was all sorts of toys and play structures for the rats. Rats were even given stimulating challenges, like solving mazes. The rats also had plenty of interaction, each cage was large enough to comfortably accommodate 10-12 rats in total. 

The rats continued in their environments for 30-60 days (different runs were conducted), after which they were euthanised (ok lab rats and mice have really given up a lot for the sake of science). A post-mortem study was then conducted to examine the brains of these rats

The rat’s brains were dissected and various sections were measured, weighed and analyzed to determine the amount of cell growth and levels of neurotransmitter activity.

And what were the results?


Rats living in the stimulating environment developed a heavier and thicker frontal cortex (the part of the brain that regulates cognition). In addition, the rats developed more acetylcholine receptors - acetylcholine is a neurotransmitter that is emitted in large amounts when we are encoding new experiences and memories into our brain. Finally, the entire weight of the brain in the simulated environment can be up to 10% heavier, with 20% more synapses or connection between neurons. This study has been replicated many times over (even till today) with very similar results.

So why is this study so important? 

This study debunked what was previously a widely held belief that the brain was fixed at birth. Rosenzweig's study showed that not only are our brains changeable due to the environment we are in, these changes can be substantial.

And we have observed the same findings when examining humans. Infants put on weight more quickly and develop more neurologically when they are handled and cuddled than when they are not.  And we have numerous studies that show how children and teenagers who have been neglected or suffer from poverty or constant stress eventually develop thinner pre-frontal cortexes (the part of our brain dealing with cognition) and a larger amygdala (regulating anxiety, anger, aggression, and fear). You might also have read Carol Dweck's terrific book on the "Growth Mindset", where students increased their IQ points and performed significantly better in a different learning environment.

Related links:

Neuroplasticity - our constantly changing brain

Neurogenesis - we can keep learning through life

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