Imagine an Italian person is living in London. A poll on the street asks them to estimate the total number of Italians in London. This person then gives a highly incorrect, overblown guess. It's simply common for people of a single nationality to notice each other in the crowd or go to the same places, so it's somewhat likely that one would have an inflated estimate in their heads.

Another example would be a "reverse physical attractiveness stereotype": a person holding the belief that "all people are friendly". However in reality this could be explained by the fact that the person holding the belief is highly attractive.

How do you call this bias? Selection bias seems close, but is there something more specific?

  • $\begingroup$ I don't think this is a bias. consider the following questions - interview questions. As you can see, they mostly ask you to estimate a number of things that are static. Number of "things" that are easy to relate to other meaninful numbers - so you can eventually narrow down your answer. Here's another resource that you can use when in an interview. But regular people don't think like this. They do not have a mind for statistics. $\endgroup$ – StefanS May 28 '16 at 21:43
  • $\begingroup$ I'd suggest you search for a list of biases then in the pages found search for "statisti" / "math". You might find something regarding this. $\endgroup$ – StefanS May 28 '16 at 21:44
  • $\begingroup$ "When people face an uncertain situation, they don’t carefully evaluate the information or look up relevant statistics. Instead, their decisions depend on a long list of mental shortcuts, which often lead them to make foolish decisions. These shortcuts aren’t a faster way of doing the math; they’re a way of skipping the math altogether. Asked about the bat and the ball, we forget our arithmetic lessons and instead default to the answer that requires the least mental effort." Source $\endgroup$ – StefanS May 28 '16 at 21:47
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    $\begingroup$ Do you have evidence for the existence of this bias? $\endgroup$ – AliceD May 28 '16 at 22:32
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    $\begingroup$ Hi @StefanSzekeres, you are very close to a nice answer. I was also thinking about availability bias. Perhaps you could rephrase you comments into an answer and provide some references with it too. $\endgroup$ – Robin Kramer May 28 '16 at 22:37

I think you're at least in part talking about the false consensus effect/bias

the false-consensus effect or false-consensus bias is an attributional type of cognitive bias whereby people tend to overestimate the extent to which their opinions, beliefs, preferences, values, and habits are normal and typical of those of others (i.e., that others also think the same way that they do). [...]

The false-consensus effect is not restricted to cases where people believe that their values are shared by the majority, but it still manifests as an overestimate of the extent of their belief. [...]

There is no single cause for this cognitive bias; the availability heuristic, self-serving bias, and naïve realism have been suggested as at least partial underlying factors.

At least your second example: "most people are X" being thought by someone who is X clearly belongs to this false-consensus-bias category.

I'm not sure your first example is quite like that though... being Italian is not something that can be reasonably assumed to be universal... but Wikipedia also mentions (2nd para quoted) that some seem apply the "false consensus bias" to describe even cases where the subjects just overestimates the extent of their belief.

The false consensus effect is thought to have some [cor]relation with the availability heuristic, or even be a causal effect of the latter... that seems to have been the ultimate conclusion of Larry's answer.

There's at least one paper starting with

Selective exposure to a biased sample of others is supposed to play an important role in explaining the false consensus effect (FCE).

And it also measured the level of correlation

Three predictions were tested. First, subjects were expected to refer to similar others more often than to dissimilar others. Second, referring to similar others was expected to lead to larger FCEs. Third, referring to dissimilar others was expected to lead to smaller FCEs. Results provided direct evidence of selective exposure, but this was most pronounced for issues for which opinions were relatively heterogeneous, i.e. for which there was no clear majority or minority opinion. The availability of similar others was further shown to be associated with larger FCEs, whereas the availability of dissimilar others was associated with smaller FCEs.

Other research suggest that there more moderators for the FCE:

Prior research has shown that individuals are often susceptible to a false consensus effect, whereby they overestimate the extent to which others share their opinions. In three studies, we show that the strength of the false consensus effect is moderated by the valence of one's own opinion, such that overestimation of population consensus is greater when an individual likes an alternative as compared to when she or he dislikes it. Further, we show that this moderation of false consensus is driven by the availability of countervalence attributes, that is, disliked attributes in liked alternatives and liked attributes in disliked alternatives.

So FCE seems more complex than being driven by mere availability, i.e. the level of like/dislike for the main characteristic as well as its associated/secondary characteristics seem to moderate FCE.

To speculate how this might influence your Italian example, subjects with a strong [and positive] ethnocentric perspective will probably exhibit more overestimation (of their percentage in the population). I don't know if anyone has tested this hypothesis though.

N.B. one study found that FCE persisted even after subjects were shown objective statistics. But the persistence of FCE is (thankfully and obviously) moderated by encountering disagreement.

Finally, one of those papers mentioned some synonyms for FCE that have been used in the literature: "assumed similarity" and "social projection".


While I am not sure if people actually do overestimate the incidence of a minority group (or maybe they do, I would want to see the data), we can treat this as a philosophical question about what we would call that phenomenon were it to exist.

The notion of bias in psychology is less precisely defined than a similar notion of bias that lives within the fields of computer science, machine learning, and computational cognitive science. This notion of bias is simply the property of an learning algorithm or a cognitive model or a person that cause it to favor some hypotheses or explanations over others in the absence of specific data. For example, if I come across an unknown large animal with big teeth, I may have a bias that causes me to expect it to attack, even though I do not have any direct experience as to whether or not it is friendly. If you think about it, any prediction we make requires such a bias, because we never have data about what has not yet happened. If you are interested in learning more about this, I suggest reading Hume, Tversky and Kahneman, and Mitchell as a start.

I agree that what you are talking about is a bias in this sense, but I do not know whether there is an established term for the specific type of bias. In general, while it is useful for exams and for reference, I recommend avoiding leaning too hard on the idea that all phenomena fit neatly into well-defined and labeled concepts such as "selection bias" or "reverse physical attractiveness stereotype." While some people might have a term for the phenomenon you have mentioned, it is more useful to articulate and understand it in terms of what it is: a cognitive tendency to overestimate subpopulations in large cities in the absence of relevant data.

Whether this is a general phenomenon about human estimation or it is limited to cases of subpopulation estimates in large cities is an empirical question. If you want to know more, you'll have to run some experiments!

Edit: Now that I think about it, this sounds like like the availability heuristic discovered by Tversky and Kahneman. Basically, the availability heuristic (or bias) is the conflation of the biased sample that you have easy access to with the full population. In the case of your example, an Italian person living in an Italian community would likely run into a lot of Italians day-to-day and therefore might come to believe that Italians are prevalent in the total population, even though this incidence of seeing Italians is solely due to his membership in this particular community.

  • $\begingroup$ Thank you for your answer. The availability heuristic is really close to what I was thinking and there's probably not a pre-defined concept for the exact concept of "ignoring the common denominator of self". $\endgroup$ – JonathanReez Jun 1 '16 at 17:04

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