<p>Since you <a href="https://cogsci.stackexchange.com/questions/1034/do-people-like-those-who-are-similar-to-them-and-why#comment2046_1035">mentioned that you want an evolutionary explanation</a>, there is one available. In biology the effect of providing benefit towards potential non-kin based on an arbitrary marker is known as the <a href="http://en.wikipedia.org/wiki/Green-beard_effect" rel="noreferrer">green-beard</a> or armpit effect. In a social human setting, if the marker is arbitrary social construct it is usually known as <a href="http://en.wikipedia.org/wiki/Ethnocentrism" rel="noreferrer">ethnocentrism</a>. This sort of behavior is studied in game theory in the context of cooperate-defect games (typical example: <a href="http://en.wikipedia.org/wiki/Prisoner%27s_dilemma" rel="noreferrer">Prisoner's dilemma</a>) and usually called <em>conditional altruism</em>.</p>
<p>It has been shown that conditional altruism evolves in a <a href="http://egtheory.wordpress.com/2012/06/13/evolution-of-ethnocentrism/" rel="noreferrer">simple spatial agent-based model</a> and promotes cooperative behavior (Hammond &amp; Axelrod, 2006a 2006b). The effect does not create cooperation, but if there is another mechanism for creation of cooperation (say spatial factors in the H&amp;A model) then ethnocentrism helps maintain it and extend the range of parameters under which cooperation can occur (Kaznatcheev &amp; Shultz, 2011).</p>
<p>In humans, this ability to cooperate only with others of similar culture is believed to require a significant amount of cognitive ability. In fact, some even suppose that it could have been one of the factors that drove towards the increasing complexity of our brains. Unfortunately, Kaznatcheev (2010a) shows that the ethnocentrism of the sort present in the H&amp;A models is not robust to increase in the cost of cognition. Thus, in humans (or simpler organisms) the mechanism allowing discrimination has to have been in place already (and not co-evolved) or be very inexpensive.</p>
<p>The above examples dealt with the prisoner's dilemma (PD) which is a typical model of a competitive environment. In the PD cooperation is irrational, so there conditional altruism allowed the agents to cooperate irrationally (thus moving over to the better social payoff), while still treating those of a different culture rationally and defection from them. This doesn't seem as bad, but Kaznatcheev (2010b) shows that the mechanism of ethnocentrism is robust across different games (not just PD) including ones where cooperation is rational. In those games conditional altruism produces an irrational defection from the out-group. Thus, from an evolutionary stand-point this is a two-edged sword: it can cause unexpected cooperative behavior, but also irrational hostility.</p>
<h3>References</h3>
<ul>
<li><p>Hammond, R., &amp; Axelrod, R. (2006a). <a href="http://www.sciencedirect.com/science/article/pii/S0040580905001656" rel="noreferrer">Evolution of contingent altruism when cooperation is expensive</a>. Theoretical Population Biology, 69, 333-338.</p>
</li>
<li><p>Hammond, R., &amp; Axelrod, R. (2006b). <a href="http://jcr.sagepub.com/content/50/6/926.short" rel="noreferrer">The evolution of ethnocentrism</a>. Journal of Conflict Resolution, 50, 926-936. (<a href="http://www.artisresearch.com/articles/Axelrod_Evolution_of_Ethnocentrism.pdf" rel="noreferrer">pdf</a>)</p>
</li>
<li><p>Kaznatcheev, A. (2010a). The cognitive cost of ethnocentrism. In S. Ohlsson &amp; R. Catrambone (Eds.), Proceedings of the 32nd annual conference of the cognitive science society. (<a href="http://www.cs.mcgill.ca/%7Eakazna/Kaznatcheev20100515.pdf" rel="noreferrer">pdf</a>)</p>
</li>
<li><p>Kaznatcheev, A. (2010b). Robustness of ethnocentrism to changes in inter-personal interactions. Complex Adaptive Systems - AAAI Fall Symposium. (<a href="http://www.cs.mcgill.ca/%7Eakazna/kaznatcheev20100910.pdf" rel="noreferrer">pdf</a>)</p>
</li>
<li><p>Kaznatcheev, A., &amp; Shultz, T.R. (2011). Ethnocentrism Maintains Cooperation, but Keeping One's Children Close Fuels It. In L. Carlson, C, Hoelscher, &amp; T.F. Shipley (Eds), Proceedings of the 33rd annual conference of the cognitive science society. (<a href="http://www.cs.mcgill.ca/%7Eakazna/KaznatcheevShultz20110501-2.pdf" rel="noreferrer">pdf</a>)</p>
</li>
</ul>

CC BY-SA 3.0
2014-02-26T17:07:57.847Z
Ooh...original research from the primary author! I feel like bounties should be awarded automatically by the community when this happens (and the research is halfway decent, of course). Speaking of which, since you're a mod now... ;)
Nick Stauner
CC BY-SA 3.0
2012-06-09T06:46:35.427Z
So the answer is it's the grease that facilitate better outcome.
user4951
CC BY-SA 3.0
2012-06-09T16:16:17.307Z
@JimThio no, it is not the "grease that facilitate better outcome". First in an evolutionary setting there is not a well defined 'better', assuming a 'better' is a teleological assumption which is frowned upon in evolutionary biology. More importantly, even with the definition of 'higher payoff for everyone' it is not better, since I specifically talk about cases where it leads to irrational defection (i.e. moving away from a Pareto dominant strategy even when it is a Nash eq.) in the last paragraph.
Artem Kaznatcheev
CC BY-SA 3.0
2012-06-08T15:45:30.433Z
2012-06-14T01:20:14.800Z
2020-06-17T09:55:31.633Z
Artem Kaznatcheev
<p>You may want to read up about "homophily". It is often summarised with the phrase "birds of a feather flock together". "Heterophily" relates to when people are attracted to those that are different to them.</p>

<p>There was a review article by McPhereson et al (2001) which you might like to read. To quote the abstract:</p>

<blockquote>
  <p>Similarity breeds connection. This principle—the homophily
  principle—structures network ties of every type, including marriage,
  friendship, work, advice, support, information transfer, exchange,
  comembership, and other types of relationship. The result is that
  people’s personal networks are homogeneous with regard to many
  sociodemographic, behavioral, and intrapersonal characteristics.
  Homophily limits people’s social worlds in a way that has powerful
  implications for the information they receive, the attitudes they
  form, and the interactions they experience. Homophily in race and
  ethnicity creates the strongest divides in our personal environments,
  with age, religion, education, occupation, and gender following in
  roughly that order. Geographic propinquity, families, organizations,
  and isomorphic positions in social systems all create contexts in
  which homophilous relations form. Ties between nonsimilar individuals
  also dissolve at a higher rate, which sets the stage for the formation
  of niches (localized positions) within social space. We argue for more
  research on: (a) the basic ecological processes that link
  organizations, associations, cultural communities, social movements,
  and many other social forms; (b) the impact of multiplex ties on the
  patterns of homophily; and (c) the dynamics of network change over
  time through which networks and other social entities co-evolve.</p>
</blockquote>

<h3>References</h3>

<ul>
<li>McPherson, M. and Smith-Lovin, L. and Cook, J.M. (2001).  Birds of a feather: Homophily in social networks. Annual review of sociology, 415--444.
<a href="ftp://www.soc.cornell.edu/csi/Networks/mcpherson%20smith-lovin%20cook%20ars.pdf" rel="nofollow">ftp://www.soc.cornell.edu/csi/Networks/mcpherson%20smith-lovin%20cook%20ars.pdf</a></li>
</ul>

CC BY-SA 3.0
2012-06-08T08:22:17.127Z
Good. So the answer is yes true. It doesn't explain, why. I mean I wonder if evolutionary psychology can provide an explanation why people are homophilic.
user4951
CC BY-SA 3.0
2012-06-08T05:28:42.347Z
2013-09-14T10:22:06.633Z
2013-09-14T10:22:06.633Z
Jeromy Anglim
<h3>Questions</h3>

<ul>
<li>Is it true that people 'like' those who are similar to them?</li>
<li>Why is it so? Is there an evolutionary explanation?</li>
</ul>

CC BY-SA 3.0
2012-06-08T16:36:08.310Z
Also, you should be careful with the word 'like' it is very ambiguous. Try to use more precise language. In particular, your question as-is could also be answered from the mate-selection perspective.
Artem Kaznatcheev
CC BY-SA 3.0
2015-07-29T21:44:17.480Z
This question echoes a proverb that "like attracts like," so the context should be clear.
Tom Au
CC BY-SA 4.0
2019-06-05T16:59:29.553Z
Similar in personality, genetic makeup, and culture.
user4951
<social-psychology><evolution>
Do people like those who are similar to them and why?
user4951
# Do people like those who are similar to them and why?

(Asked by: username_0 on 2012-06-08)

### Questions

* Is it true that people 'like' those who are similar to them?
* Why is it so? Is there an evolutionary explanation?

**Tags:** social-psychology, evolution

> **Comments:**
> **username_1**: Also, you should be careful with the word 'like' it is very ambiguous. Try to use more precise language. In particular, your question as-is could also be answered from the mate-selection perspective.  
> **username_2**: This question echoes a proverb that "like attracts like," so the context should be clear.  
> **username_0**: Similar in personality, genetic makeup, and culture.  

---

## 2 answers

### Answer by username_4 on 2012-06-08. Score: 10 | Accepted answer

You may want to read up about "homophily". It is often summarised with the phrase "birds of a feather flock together". "Heterophily" relates to when people are attracted to those that are different to them.

There was a review article by McPhereson et al (2001) which you might like to read. To quote the abstract:

> Similarity breeds connection. This principle—the homophily
> principle—structures network ties of every type, including marriage,
> friendship, work, advice, support, information transfer, exchange,
> comembership, and other types of relationship. The result is that
> people’s personal networks are homogeneous with regard to many
> sociodemographic, behavioral, and intrapersonal characteristics.
> Homophily limits people’s social worlds in a way that has powerful
> implications for the information they receive, the attitudes they
> form, and the interactions they experience. Homophily in race and
> ethnicity creates the strongest divides in our personal environments,
> with age, religion, education, occupation, and gender following in
> roughly that order. Geographic propinquity, families, organizations,
> and isomorphic positions in social systems all create contexts in
> which homophilous relations form. Ties between nonsimilar individuals
> also dissolve at a higher rate, which sets the stage for the formation
> of niches (localized positions) within social space. We argue for more
> research on: (a) the basic ecological processes that link
> organizations, associations, cultural communities, social movements,
> and many other social forms; (b) the impact of multiplex ties on the
> patterns of homophily; and (c) the dynamics of network change over
> time through which networks and other social entities co-evolve.

### References

* McPherson, M. and Smith-Lovin, L. and Cook, J.M. (2001). Birds of a feather: Homophily in social networks. Annual review of sociology, 415--444.
  <ftp://www.soc.cornell.edu/csi/Networks/mcpherson%20smith-lovin%20cook%20ars.pdf>

> **Comments:**
> **username_0**: Good. So the answer is yes true. It doesn't explain, why. I mean I wonder if evolutionary psychology can provide an explanation why people are homophilic.  

---

### Answer by username_1 on 2012-06-08. Score: 13

Since you [mentioned that you want an evolutionary explanation](https://cogsci.stackexchange.com/questions/1034/do-people-like-those-who-are-similar-to-them-and-why#comment2046_1035), there is one available. In biology the effect of providing benefit towards potential non-kin based on an arbitrary marker is known as the [green-beard](http://en.wikipedia.org/wiki/Green-beard_effect) or armpit effect. In a social human setting, if the marker is arbitrary social construct it is usually known as [ethnocentrism](http://en.wikipedia.org/wiki/Ethnocentrism). This sort of behavior is studied in game theory in the context of cooperate-defect games (typical example: [Prisoner's dilemma](http://en.wikipedia.org/wiki/Prisoner%27s_dilemma)) and usually called *conditional altruism*.

It has been shown that conditional altruism evolves in a [simple spatial agent-based model](http://egtheory.wordpress.com/2012/06/13/evolution-of-ethnocentrism/) and promotes cooperative behavior (Hammond & Axelrod, 2006a 2006b). The effect does not create cooperation, but if there is another mechanism for creation of cooperation (say spatial factors in the H&A model) then ethnocentrism helps maintain it and extend the range of parameters under which cooperation can occur (Kaznatcheev & Shultz, 2011).

In humans, this ability to cooperate only with others of similar culture is believed to require a significant amount of cognitive ability. In fact, some even suppose that it could have been one of the factors that drove towards the increasing complexity of our brains. Unfortunately, Kaznatcheev (2010a) shows that the ethnocentrism of the sort present in the H&A models is not robust to increase in the cost of cognition. Thus, in humans (or simpler organisms) the mechanism allowing discrimination has to have been in place already (and not co-evolved) or be very inexpensive.

The above examples dealt with the prisoner's dilemma (PD) which is a typical model of a competitive environment. In the PD cooperation is irrational, so there conditional altruism allowed the agents to cooperate irrationally (thus moving over to the better social payoff), while still treating those of a different culture rationally and defection from them. This doesn't seem as bad, but Kaznatcheev (2010b) shows that the mechanism of ethnocentrism is robust across different games (not just PD) including ones where cooperation is rational. In those games conditional altruism produces an irrational defection from the out-group. Thus, from an evolutionary stand-point this is a two-edged sword: it can cause unexpected cooperative behavior, but also irrational hostility.

### References

* Hammond, R., & Axelrod, R. (2006a). [Evolution of contingent altruism when cooperation is expensive](http://www.sciencedirect.com/science/article/pii/S0040580905001656). Theoretical Population Biology, 69, 333-338.
* Hammond, R., & Axelrod, R. (2006b). [The evolution of ethnocentrism](http://jcr.sagepub.com/content/50/6/926.short). Journal of Conflict Resolution, 50, 926-936. ([pdf](http://www.artisresearch.com/articles/Axelrod_Evolution_of_Ethnocentrism.pdf))
* Kaznatcheev, A. (2010a). The cognitive cost of ethnocentrism. In S. Ohlsson & R. Catrambone (Eds.), Proceedings of the 32nd annual conference of the cognitive science society. ([pdf](http://www.cs.mcgill.ca/%7Eakazna/Kaznatcheev20100515.pdf))
* Kaznatcheev, A. (2010b). Robustness of ethnocentrism to changes in inter-personal interactions. Complex Adaptive Systems - AAAI Fall Symposium. ([pdf](http://www.cs.mcgill.ca/%7Eakazna/kaznatcheev20100910.pdf))
* Kaznatcheev, A., & Shultz, T.R. (2011). Ethnocentrism Maintains Cooperation, but Keeping One's Children Close Fuels It. In L. Carlson, C, Hoelscher, & T.F. Shipley (Eds), Proceedings of the 33rd annual conference of the cognitive science society. ([pdf](http://www.cs.mcgill.ca/%7Eakazna/KaznatcheevShultz20110501-2.pdf))

> **Comments:**
> **username_0**: So the answer is it's the grease that facilitate better outcome.  
> **username_1**: @JimThio no, it is not the "grease that facilitate better outcome". First in an evolutionary setting there is not a well defined 'better', assuming a 'better' is a teleological assumption which is frowned upon in evolutionary biology. More importantly, even with the definition of 'higher payoff for everyone' it is not better, since I specifically talk about cases where it leads to irrational defection (i.e. moving away from a Pareto dominant strategy even when it is a Nash eq.) in the last paragraph.  
> **username_3**: Ooh...original research from the primary author! I feel like bounties should be awarded automatically by the community when this happens (and the research is halfway decent, of course). Speaking of which, since you're a mod now... ;)  

---

