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Shit scared

BMJ 2003; 326 doi: (Published 01 June 2003) Cite this as: BMJ 2003;326:0306204
  1. Ayesha Nunhuck, third year medical student1
  1. 1Barts and the Royal London Hospital

Animals do not have a problem with it. Small children even play with it. So why are adult humans so disgusted by faeces? Ayesha Nunhuck investigates

Until my second year as a medical student, I was only familiar with aversion to faeces as some sort of humour. But the aversion is widespread: why are we so disgusted by our own excrement?

An evolutionary advantage?

Valerie Curtis of the London School of Hygiene and Tropical Medicine believes rejection of bodily secretions, such as faeces, may be a biological mechanism for avoiding infectious disease (Anatomy of Disgust, Channel 4, 2000). More than 1011 bacteria are found in each gram of dry faeces; 40%-50% of the dry weight.13 Also, infected individuals excrete high numbers of viruses, such as cholera and typhoid.45 Generally, contamination by human excrement is regarded as the greatest risk to water supplies, with a huge resultant morbidity and mortality worldwide.56

When and how humans became averse to faeces is not known. One proposal is that our ancestors distancing themselves from faeces was to aid survival by concealing their scent from predators.7 Bacteria in the intestine and faeces ferment carbohydrate residues releasing volatile fatty acids and pungent compounds such as ammonia and sulphur-rich compounds such as hydrogen sulphide, mercaptans, indoles, and skatole. Indoles and skatole give excrement its most characteristic odour.23 The sulphur rich compounds are expecially volatile and can be sensed immediately--this supports the argument that ancestors hid their trail from predators by defaecating into back filled pits.7

Magnetic resonance imaging studies have shown that the anterior insular cortex in the brain is responsible for disgust and is believed …

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