Pigs Can Be Optimistic or Pessimistic Depending on Both Environment and Personality

A team of researchers has found that domestic pigs can express both optimism and pessimism. Optimistic pigs were better at adapting to less ideal environments and were more likely to seek out possible rewards. This was the first time that optimistic and pessimistic personality traits were demonstrated in a nonhuman animal. The details are in a paper that was just published in the journal Biology Letters.

When humans are in a negative mood, they display pessimism and are more likely to assume the worst outcome of a situation. If someone is in a positive, optimistic mood, they are more likely to expect a better outcome. This is called cognitive bias and it happens even when both outcomes are equally likely. Scientists have studied this phenomenon in humans but not with other animals.

Researchers studied cognitive biases and personality in domestic pigs (Sus scrofa domesticus). The team tested 36 individuals and started the study by observing how the pigs reacted to unknown objects. Pigs that were cautious and active were labeled as “reactive” and pigs that were curious and flexible were labeled as “proactive”. The team then set up a scenario in which there were two bowls, one with tasty chocolates and one with bitter coffee beans. The researchers coated the food with sugar to make it difficult for the pigs to tell the difference based on scent. As time went on, the bowls were moved around, sometimes to the middle of the room.

The pigs were also split into two groups; some lived in standard but still comfortable pens while the others lived in enriched pens with more room and lots of cozy straw. The pigs were then released into an area with a bowl in the middle that could contain either the coffee beans or the chocolates.

Pigs with “reactive” personalities were more likely to investigate the bowl if they had lived in the deluxe pen, otherwise they left the bowl alone. The proactive pigs, on the other hand, investigated the bowl regardless of accommodations and overall showed more optimism. This shows that pigs can be both optimistic and pessimistic, with some of it based on personality and not simply environment.

The team’s findings show the existence of optimism and pessimism in pigs based on personality. This might explain inconsistent results in previous animal studies; the researchers hadn’t taken individual personalities into account. For the first time, scientists have evidence of optimistic and pessimistic personality traits in a nonhuman animal.


Asher et al. Mood and personality interact to determine cognitive biases in pigs. Biology Letters (2016).

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