Cues that are received from other individuals can potently alter future behavior and the brain mechanisms that control that behavior. For an example from an animal model, consider the fact that rats spend a lot of time investigating the smells around them, including those coming from other rats. Cooke et al. (2000) took young rats, just weaned from their mother, and either raised each male in a cage alone, or raised them with other males to play with. Examination of these animals as adults found only one brain difference between the groups: a region of the brain known to process odors was smaller in the isolated males than in the males raised with playmates (see Figure 1). Was it the lack of play (Gordon et al., 2003), the lack of odors to investigate, or the stress of isolation that made the region smaller? Whatever the mechanism, social experience affects this brain structure. It’s an idea that we return to in several places in the book.

Figure 1  The Role of Play in Brain Development
A brain region involved in processing odors (the posterodorsal portion of the medial amygdala) was smaller in male rats housed individually compared to males housed together and allowed to play. Other nearby regions were identical in the two groups. (After Cooke et al., 2000.)

Here’s an example of how social influences can affect the human brain. When people were asked to put a hand into moderately hot water (47°C [117°F]), part of the brain became active, presumably because of the discomfort involved (Rainville et al., 1997). But subjects who had been led to believe the water would be very hot had a more activated brain than did subjects who had been led to believe the discomfort would be minimal (see Figure 2), even though the water was the same temperature for all subjects. The socially induced psychological expectation affected the magnitude of the brain response, even though the physical stimulus was exactly the same. (By the way, the people with the more-activated brains also reported that their hands hurt more.)

Figure 2  Pictures of Pain
Subjects who were told to expect only mild discomfort from putting a hand into 47°C water (left) showed less activation in a particular brain region (the anterior cingulate cortex) than did subjects expecting more discomfort (right) from water of the very same temperature. Areas of high activation are indicated by orange, red, and white. (From Rainville et al., 1997; courtesy of Pierre Rainville.)


Cooke, B. M., Chowanadisai, W., and Breedlove, S. M. (2000). Post-weaning social isolation of male rats reduces the volume of the medial amygdala and leads to deficits in adult sexual behavior. Behavioural Brain Research 117: 107–113.

Gordon, N. S., Burke, S., Akil, H., Watson, S. J., et al. (2003). Socially-induced brain “fertilization”: Play promotes brain derived neurotrophic factor transcription in the amygdala and dorsolateral frontal cortex in juvenile rats. Neuroscience Letters 341: 17–20.

Rainville, P., Duncan, G. H., Price, D. D., Carrier, B., et al. (1997). Pain affect encoded in human anterior cingulate but not somatosensory cortex. Science 277: 968–971.