In sociology, a friend of a friend is a human contact that exists because of a mutual friend. Person C is a friend of a friend of person A when there is a person B that is a friend of both A and C. Thus the human relation "friend of a friend" is a compound relation among friends, similar to the uncle and aunt relations of kinship. Though friendship is a reciprocal relation, the relation of a friend of a friend may not be a friendship, though it holds potential for coalition building and dissemination of information.
在社会学（Sociology）中，朋友的朋友是因为共同的朋友而存在的人际关系。当甲和丙都是乙的朋友，丙就成为甲的朋友的朋友。因此，人际关系中的“朋友的朋友”是朋友之间的复合关系，类似于亲属关系(kinship)中的舅舅与舅妈关系（舅妈如果没有成为你舅舅的妻子，就不会成为你的舅妈）。虽然友谊是一种对称关系｜相互关系(symmetric relation|reciprocal relation)，但朋友的朋友的关系可能不是友谊，尽管它具有建立联盟和传播信息的潜力。
The tendency of a friend of a friend to become a friend was noted by Fritz Heider, though he also considered the possibility that one of the friendships might breakdown, according to balance theory, which his view of human triangles is called. According to Heider, the friend of a friend contact could be stressful enough to undermine one or another of the friendships. Extending the study of social dynamics caused by such friend-of-a-friend tensions to social networks beyond triangles, D. Cartwright and Frank Harary used signed graphs to indicate positive or negative sentiments between persons. In 1963 Anatol Rapoport summarized the theory: "The hypothesis implies roughly that attitudes of the group members will change is such a way that one's friends' friends will tend to become one's friends, ..." In September 1975 Dartmouth College offered a symposium on these dynamics.
弗里茨·海德(Fritz Heider) 注意到了朋友的朋友会成为朋友的倾向，根据他的平衡理论（他对人类的三角关系的观点被称为平衡理论），他也考虑了三者之间友谊有破裂的可能性。根据海德的说法，与朋友之友的社会关系，可能会有足够的压力，来破坏这两者或那两者之间的友谊关系。卡特赖特(D. Cartwright)和弗兰克·哈拉里(Frank Harary)将这种朋友之友的紧张关系（三角关系）引起社会动力学的研究，扩展到三角关系之外的社会关系网络，他们使用符号图(signed graph)表示人与人之间的积极或消极情绪。1963年，阿纳托尔·拉波波特（Anatol Rapoport） 总结了这一理论:“这个假设大致意味着群体成员的态度会发生改变，也就是，一个人的朋友之友会成为他的（直接的）朋友...”1975年9月，达特茅斯学院（Dartmouth College）举办了一个关于这类动力学的研讨会 。
Bo Anderson made an analysis of the friend-of-a-friend relationship in connection with his criticism of balance theory.
We have all encountered cases in which somebody has said, "You should meet so-and-so", only to find that we have little in common with that person, even though he or she was introduced to us by a mutual friend...In some friendships the persons value the exclusiveness of their relationship and are therefore not likely to let others into it. Friends differ from acquaintances in that they are not merely slots in a grid of social network relationships, but are valued for their personal, unique qualities. Hence, when I relate to a friend of a friend, I need to know something about the perceptions and exchanges that make up this friendship. My reaction to my friend's friend (or spouse) may even be unfavorable, although I may also well understand and sympathize with my friend’s affection for her, given his needs, perceptions, interests and so on.
我们都遇到过这样的情况，有人说: “你应该见见某某” ，结果却发现我们和那个人没有什么共同点，尽管他或她是由彼此共同的朋友介绍给我们的... ... 在“某些”友谊关系中，人们看重他们关系的排他性，因此不太可能让别人介入。朋友与熟人的不同之处在于，朋友不仅仅是社交网络关系网中的一个网格的占位，而且因其个人的独特品质而受到重视。因此，当我与朋友的朋友建立联系时，我需要了解构成这种友谊的看法，并相互交流。我对朋友的朋友(或配偶)的反应，甚至可能是“不利的”，尽管我考虑到朋友的需要、看法、兴趣等等之后，“也”能理解和同情我的朋友对她的感情。
Considering friendship between people to be a binary relation, the connection to a friend of a friend is a composition of the relationship with itself. Composed relations are used to describe kinship, so it may be natural to apply composition to friendship. One consequence is that frequently a person's friends have more friends than him (the friendship paradox), which accents the reach of the compound connection. But the fact that friendship is not automatically a transitive relation produces some social dynamics.
考虑到人与人之间的友谊(friendship)是一种两人关系(binary relation)，与朋友的朋友的联系，从关系本身而言，是一种复合关系(composition of relations)。复合关系是用来描述亲属关系#复合关系(kinship#Composition of relations)的，所以，把这种“复合”用到友谊上，是自然而然的。一个后果是，一个人的朋友，经常比这个人有更多的朋友（友谊悖论），这突出了复合关系的影响范围。但事实上，友谊并不会自动成为一种传递关系(transitive relation)，这就产生了某些社会动力学。
Friend of a friend (FOAF) is a phrase used to refer to someone that one does not know well, literally, a friend of a friend.
In some social sciences, the phrase is used as a half-joking shorthand for the fact that much of the information on which people act comes from distant sources (as in "It happened to a friend of a friend of mine") and cannot be confirmed. It is probably best known from urban legend studies, where it was popularized by Jan Harold Brunvand.
- "Dúirt bean liom go ndúirt bean léi" (Irish proverb) – similar Irish language term literally meaning a woman told me that a woman told her that...
- "L'homme qui a vu l'homme qui a vu l'ours" (French proverb) – similar French language proverb literally meaning The man who saw the man who saw the bear, in which the bear is never seen, only heard of.
- "Un amigo me dijo que un amigo le dijo..." (Spanish proverb) – meaning literally A friend told me that a friend told him that...
- "Jedna paní povídala..." (Czech proverb) – similar Czech language proverb literally meaning One lady said...
- "Teman kepada teman saya..." Bahasa Indonesia; literally meaning friend of my friend.
- "Babaturana babaturan urang..." Basa Sunda; literally meaning friend of my friend. There is another version of this phrase in Sundanese language, "Babaturan dulur urang", which means "friend of my relatives".
- "Diz-se que..." or "Dizem que..." Portuguese; literally meaning It is said that... or They say that...
- "Freundesfreund" German; literally meaning a friend's friend
- "Umgani womgani wami" IsiZulu language; meaning my friend's friend
网络本体语言Web ontology language
In information science, an ontology describes categories, properties and relations between concepts, data and entities. The phrase "Friend Of A Friend", converted to the acronym FAOF, has been adopted in Web Ontology Language. It has been used in WebID for identifying correspondents, and to designate a secure authentication protocol.
|40x40px||Look up FOAF in Wiktionary, the free dictionary.|
- 个体之间的联系（Interpersonal ties）
- 六度分割(Six degrees of separation)
- Fritz Heider (1946) "Attitudes and Cognitive Organization", The Journal of Psychology 21: 107 to 21
- Cartwright, D. and Frank Harary (1956)Structural balance: a generalization of Heider's theory, Psychological Review 63: 277–293 link from Stanford University
- Anatol Rapoport (1963) "Mathematical models of social interaction", in Handbook of Mathematical Psychology, v. 2, pp 493 to 580, especially 541, editors: R.A. Galanter, R.R. Lace, E. Bush, John Wiley & Sons
- Paul W. Holland & Samuel Leinhardt (editors) (1979) Perspectives on Social Network Research, Academic Press
- Bo Anderson (1979) "Cognitive Balance Theory and Social Network Analysis: Remarks on some fundamental theoretical matters", pages 453–69 in Perspectives on Social Network Research, editors: Paul W. Holland & Samuel Leinhardt, Academic Press, see page 458.
- Goodreau SM, Kitts JA, Morris M (2009). "Birds of a feather, or friend of a friend? Using exponential random graph models to investigate adolescent social networks". Demography. 46 (1): 103–25. doi:10.1353/dem.0.0045. PMC 2831261. PMID 19348111.
- Brunvand, Jan Harold (2012). Encyclopedia of Urban Legends, Updated and Expanded Edition. ABC-CLIO. p. 241. https://books.google.com/books?id=9xOb-19lXx8C&pg=PA241.
- "L'homme qui a vu l'homme qui a vu l'ours". Retrieved 14 February 2018.
- "Movie Reviews". 13 February 2018. Retrieved 14 February 2018 – via NYTimes.com.
- Foaf+ssl at W3.org