Optimism Bias

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True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Relative Difference
Human Universality: 
Individual Universal (All Individuals Everywhere)
MOCA Domain: 
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   Although rationality is often presented as the pinnacle of human cognitive abilities, the behavior of our species often departs from rational expectations. One of the most well established deviations from objective rationality is that a hallmark of a healthy human psyche is a bias towards optimism and overconfidence. We consistently over-estimate the probability of positive future events such as long life, professional success, and raising talented children, while discounting the probability of the negative, like divorce, car accidents or health problems. Further, a vast majority of individuals rate their abilities as “above-average,” a statistical impossibility, for everything ranging from attractiveness, to competency, to driving abilities. Sharot has suggested that this optimism bias is maintained by a bias in how our brains integrate evidence, with a more efficient and faithful incorporation of positive evidence, relative to negative. Sharot further implicates areas of the inferior frontal gyrus as critical for maintaining this biased information integration. With only a little controversy, this optimism bias is accepted as a human universal, spanning all ages, genders and cultures, with one important exception. Individuals with mild or severe depression show a loss or reversal of the optimism bias, respectively. Varki has suggested that an ability to deny negative reality, namely our own mortality, was a critical step in allowing our species to develop the level self-awareness unique to humans.
   Beyond the proximate benefits of longer life, improved health and greater professional and financial success seen in those with an optimist outlook, there are at least three interrelated evolutionary explanations for maintaining a positive illusion as a cognitive bias. Johnson and Fowler suggest that in a world of competitions, over-estimating one’s ability to win competitions would lead to increased fitness by increasing participation in contests. In a related note, Triver’s suggested that when deception is an efficient strategy, it is enhanced when the individual themselves believes the deception. Thus an optimism bias may then more effectively signal formidability and worth. Finally, Haselton invokes Error Management Theory to suggest that positive illusions such as an optimism bias should be expected when the cost of investing effort is low and the potential pay-off is high. In this case the positive illusions would serve to maintain effort in beneficial goal directed behavior.
   Studying an optimism bias across species is a challenge, as it requires one to investigate the beliefs and motivations of non-human animals. While little work has been done in other great apes, a number of studies have suggested that, much like the human depression, inducing negative and positive affect can induce relatively pessimistic or optimistic biases. While these experiments have shown that rats, dogs, rhesus monkeys, pigs, bees and chickens can acquire an optimistic bias, as these experiments are performed in laboratory settings it is unclear how and if an optimistic bias is present at baseline in the wild.


Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 
150,000 thousand years ago
Probable Appearance: 
30,000 thousand years ago
Definite Appearance: 
200 thousand years ago
The Human Difference: 

 While it has been suggested that a number of species can acquire a relatively optimistic or pessimistic bias it has been argued by Varki and Sharot independently that humans are born with an innate, hard-wired bias towards optimism. At current there is little evidence and few strong arguments made about a similar innate bias towards optimism in other species, although this is impart due to a lack of rigorous testing.

Universality in Human Populations: 

 There have been a few studies that examined the presence of an optimism bias across cultures, and while there are some reports of an absence in certain cultures, later work suggested that a bias towards optimism was present in these cultures, but in different domains. The strongest consensus is that an optimistic bias appears to be a human universal, although there is cultural variability in the particular domains which demonstrate an optimistic bias.

Possible Selection Processes Responsible for the Difference: 

There have been multiple models proposed for the selection of an optimism bias. Most of these suggest a role of motivating goal directed behavior, deceptive signaling to others or engagement in risky competition. Varki has also suggested that a particular type of optimism bias, that of denying our own mortality, would be required for the successful evolution of knowledge of our own mortality.

Implications for Understanding Modern Humans: 

 Understanding the roots of our optimism bias has important implications for understanding the behavior and evolution of modern humans. The field of behavioral economics has emerged as an attempt to understand how and when humans depart from rationality. A better understanding of our cognitive biases has important implications in a variety of domains.


  1. The evolution and psychology of self-deception., von Hippel, William, and Trivers Robert , Behav Brain Sci, 02/2011, Volume 34, Issue 1, p.1-16; discussion 16-56, (2011)
  2. The evolution of overconfidence., Johnson, Dominic D. P., and Fowler James H. , Nature, 2011 Sep 15, Volume 477, Issue 7364, p.317-20, (2011)
  3. The optimism bias., Sharot, Tali , Curr Biol, 2011 Dec 6, Volume 21, Issue 23, p.R941-5, (2011)
  4. Cognitive bias as an indicator of animal emotion and welfare: Emerging evidence and underlying mechanisms, Mendl, M., Burman O. H. P., Parker R. M. A., and Paul E. S. , Special Issue: Animal Suffering and Welfare, 05/2009, Volume 118, Issue 3–4, p.161 - 181, (2009)
  5. The paranoid optimist: an integrative evolutionary model of cognitive biases., Haselton, M. G., and Nettle D. , Pers Soc Psychol Rev, Volume 10, Issue 1, p.47-66, (2006)
  6. Illusion and well-being: a social psychological perspective on mental health., Taylor, S E., and Brown J D. , Psychol Bull, 03/1988, Volume 103, Issue 2, p.193-210, (1988)