
Friends are as genetically similar as fourth cousins - gwern
http://www.pnas.org/content/111/Supplement_3/10796.long
======
roninb
In case you were also wondering what a "fourth cousin" was, someone who shares
a pair of great-great-great grandparents[0]. Wikipedia has a great set of
diagrams[1] in case you're also confused about first, second, or third
cousins. I figure through induction you can presume any nth and nth removed
from that page (I foolishly thought 3rd was where people arbitrarily stopped
keeping count....)

[0] [https://www.reference.com/family/fourth-
cousin-7b40e096c6d65...](https://www.reference.com/family/fourth-
cousin-7b40e096c6d654b7)

[1]
[https://en.wikipedia.org/wiki/Cousin#First_cousins](https://en.wikipedia.org/wiki/Cousin#First_cousins)

~~~
fiatjaf
I get mad at people who think sons of first-cousins are second-cousins all the
time -- and, frankly, almost everyone I know think this way.

I'm glad you linked to this Wikipedia article that proves I'm not mad into
thinking there is a simple logic pattern that can be applied to cousinship.

However, this "once removed" notation is crazy. Why not second-uncle for
parents of second-cousins and so on?

~~~
dietrichepp
Pardon me for my ignorance, but wouldn't sons of first cousins be second
cousins?

~~~
ironrabbit
Language quirk :) The son of my first cousin is my first cousin once removed.
My son and my cousin's son are second cousins.

~~~
hellofunk
Actually the term "second cousin" does not have a concrete meaning. It can
mean a true second cousin, or it can also mean a generation removed from a
first cousin. The "once removed" terminology was introduced to compensate for
this ambiguity.

------
TheLarch
The lead author, Nicholas Christakis, is the man who was reviled by some
students at Yale during the Halloween costume debacle:
[http://www.theatlantic.com/politics/archive/2015/11/the-
new-...](http://www.theatlantic.com/politics/archive/2015/11/the-new-
intolerance-of-student-activism-at-yale/414810/).

~~~
sliverstorm
I hope you're not suggesting that has any bearing on the quality of the paper?

~~~
TheLarch
Not sure if you are speculating that I am a supporter or detractor of his. My
only impression of the man up until now was that I thought he and his wife
conducted themselves with grace and forebearance during the Halloween fiasco.
I find this research fascinating.

------
douche
Where I grew up, it's a pretty good chance all my friends _were_ fourth or
fifth cousins. Small population + limited outbreeding = family hedges, instead
of family trees.

There were kids in my grade who couldn't date in town because all the girls
were their first or second cousins...

~~~
mnbcvx
> _second cousins_

Didn't stop my great grandparents :) My mom's side is Mennonite, which is also
a very tight knit community. When my grandma sends random pictures from in and
around Goshen, Indiana, my wife has commented multiple times "whoa that guy in
the background really looks like you!"

~~~
runholm
Really not an issue with second cousins. Second cousins are genetically
different enough that issues with the children are just barely more common in
non-related couples.

~~~
mnbcvx
Yep. That's the kind of thing you look up at a young age when you discover
you're the recent product of second cousins. Some interesting quotes from the
"Cousin Marriage" Wikipedia article:

 _" According to Professor Robin Fox of Rutgers University, it is likely that
80% of all marriages in history may have been between second cousins or
closer."_

 _" Worldwide, more than 10% of marriages is between first and second
cousins."_

 _" In some countries it is seen as incestuous and is legally prohibited: it
is banned in China and Taiwan, the majority of U.S. states, North Korea and
South Korea."_

[https://en.wikipedia.org/wiki/Cousin_marriage](https://en.wikipedia.org/wiki/Cousin_marriage)

~~~
hellofunk
Einsten, Darwin, and Giuliani all married their _first_ cousins, from what I
understand.

~~~
pliny
That is less interesting, in the context of inbreeding, than any of them being
products of first cousin marriage.

~~~
eru
I think Roentgen, discoverer of x-rays, was the product of first cousins.

(I bet Wikipedia has a list of famous persons being children of first
cousins.)

------
astazangasta
This is a dataset designed to study genetics in a cohort of mostly white
people from Framingham, MA. How can anyone claim to be able to draw
conclusions about the genetics of friendship from this cohort? This is such a
biased dataset I can't believe this got published.

For the record, I am Indian, my best friend growing up was Italian, my current
best friends are a Russian Jew, a Scotch Irish mix from Woburn, and a Canadian
of some uncertain European temperament. I have never had a best friend even
close to being my fourth cousin.

~~~
qq66
The genes that you get from your ethnicity are a small fraction of your
overall genome. There are tons of other genes that affect things like your
height, your interest in music, etc, that could strongly affect your friend
selection.

~~~
astazangasta
And I got those in some other way than my ancestry...?

------
ilaksh
Could it just be more common for people who are in the same social group to be
in the same ethnic group, and maybe that explains most of the genetic
similarity they saw, rather than necessarily proving some kind of fitness
advantage or something? Probably because often your friends are from the same
neighborhood.

~~~
aab0
The genetic similarity here is estimated within the cohort; the Framingham
cohort is by design ethnically homongeous to try to eliminate that sort of
population structure confound. This way, the homophily is not picking up on
the obvious stuff like ethnic groups. If they studied people from multiple
ethnic groups, the results would be trivial and uninteresting. (This is why
astazangasta's criticism in another comment is so amusing - this is meaningful
_because_ the sample is so 'biased'. This also holds for all the other studies
done on Framingham.) But because only one highly homogeneous sample is
studied, the chance genetic similarities of friends aren't due to just similar
ethnic backgrounds, but are being caused by the genetic influence on things
like SES and personality and intelligence and religiosity and hobbies and
things like that. These similarities would probably also hold when considering
a more unusual and cosmopolitan group, but it would be difficult to spot the
increased similarity on a genome-wide basis because of all the racial
differences in genomes (most of which would be nonfunctional and irrelevant to
anything).

~~~
a_bonobo
They only apply population stratification from Figure 2 including onwards:

>The results so far do not control for population stratification because we
wanted to characterize overall similarity. However, it is important to
remember that some of the similarity in genotypes can be explained by simple
assortment into relationships with people who have the same ancestral
background. The Framingham Heart Study is composed of mostly whites (e.g., of
Italian descent), so it is possible that a simple preference for ethnically
similar others could explain the results in Fig. 1. However, in the following
results, we applied strict controls for population stratification to ensure
that any correlation we observed was not due to such a process.

I don't understand why they did not do this for Figure 1 (or why they do not
show that figure) - I assume that the difference disappeared, but who knows.

I just realized that this paper is from 2014, first published on arxiv in
2013. I can't find any study replicating it in the 34 citations since so I
would be careful (but this type of dataset for replication is hard to come by)
[https://scholar.google.com.au/scholar?cites=7301900347564089...](https://scholar.google.com.au/scholar?cites=7301900347564089851&as_sdt=2005&sciodt=0,5&hl=en)

------
LoSboccacc
* fourth cousins genetically as diverse as random strangers would be a better wording

the current title has implications not yet rigorously tested and people is
already looking into selection bias to explain correlation where probably
there's none.

~~~
tomtomtom777
> fourth cousins genetically as diverse as random strangers would be a better
> wording

That is not the same at all. From the abstract:

> More than any other species, humans form social ties to individuals who are
> neither kin nor mates, and these ties tend to be with similar people.

They specifically tested _friends_ to include the general preference of
friends to be similar.

------
thesz
Related:
[https://www.sciencedaily.com/releases/2008/02/080207140855.h...](https://www.sciencedaily.com/releases/2008/02/080207140855.htm)

Third cousins are most fertile.

So, marrying friend is a good idea from fertility point of view.

~~~
amelius
"Fertility" does not necessarily mean "fit offspring", but your point stands.

~~~
thesz
Define "fit offspring".

I consider "offspring that has many offsprings" as one that fits definition
"fit offspring". If you read the article you would know that third cousins'
couples have more grandsons. Thus, third cousins direct offsprings are... more
fit than others!

------
sndean
There's a pretty good CGP Grey video that explains family trees and what
fourth cousins are:

[https://www.youtube.com/watch?v=PM79Epw_cp8](https://www.youtube.com/watch?v=PM79Epw_cp8)

------
compactmani
The main finding from my perspective is that we can score and rank friendship
likeliness and see signal in a hold out dataset.

I can almost hear Facebook clamoring to offer free genotyping.

~~~
Snoooze
The dating world is already experimenting. (e.g.
[http://www.genepartner.com/](http://www.genepartner.com/))

------
cafard
How many Americans know any fourth cousins? In the village from which two
great grandparents emigrated, I have met some third cousins, some at least
twice removed, and here in the US I have met second cousins once removed. For
what it's worth, it would take a very sharp eye, and maybe an unduly
imaginative one, to spot family resemblances between the third cousins and me.

~~~
tomrod
I know plenty, due to both an active family reunion group and due to
genealogical research. It can be pretty fun learning about your ancestors.

------
peter303
Most people with European ancestry are 10th or 11th cousins to each other
based upon how every US president is related to every other. This includes the
current President who is half Eoropean.

~~~
Retric
2 ^ 11 = 2048 great ... grandparents. However, the top row only counts as one
set. Thus, at most 1,024 couples. 10 generations down with 3 kids a generation
is 59,049. Assuming zero deaths and zero intermingling that's 60,466,176.

So, your going to need very large family sizes for that to work out once you
account for deaths without children or intermingling. France averaged 4.5 kids
a couple in 1800 so I don't think this math works.

~~~
peter303
Its not math, but exhaustive geneologies by experts who've tracked every US
President as far back as Saxon England. These emperical results constrain
theoretical calculations.

~~~
Retric
Your assuming US Presidents are a representative sample. That seems highly
dubious. I mean one is the child of another. More importantly 11th century is
much more than 11 generations back. Go to 20 generations and include people at
different generations then sure.

------
avodonosov
Can it be interpreted that friends are as similar as random people?

~~~
Snoooze
No. The article mentions a number of reasons why friends can be (and are)
genetically closer than random people.

------
peter303
Assuming you have 44,000 genes (a copy from each parent) and random
recombination each generation, then some of your ancestors will have stoped
contributing genes by 16 generations (400 years). These assumptions arent
perfect, but givebyou a sense of what happens.

(People have far fewer than 2^N ancestors due to distant cousin inbreeding)
(Genetic recombination is not random, but chunky.)

~~~
Retric
This is not necessarily accurate as a fair amount of what we think of as Junk
DNA is actually used. And a large chunk of DNA is devoted to basic cellular
function which fails if it's modified.

------
aj7
Very weak correlations. Spun as a hypothesis-positive result.

------
jlebrech
so what would be the best way to increase genetic diversity?

~~~
monkpit
Why are you under the impression that it needs to be increased?

------
bencollier49
Doesn't 23andme "identify" fourth cousins?

------
rogerthis
Haven't read the text yet, but the title remembered me of the Hungarian
psychiatrist Leopold Szondi.

------
awl130
in other words, we pick friends from the same ethnic group

------
microcolonel
I live in Toronto, and frankly I doubt my Panjabi, Sri Lankan, Chinese, and
Korean friends are as genetically similar as my fourth cousins.

~~~
eggie
It could be that you share certain alleles at a much higher rate than expected
by chance.

That said, if the study is working off the Framingham data, I do wonder how
general a conclusion it is...

------
kbd
Pretty sure my Thai best friend is more dissimilar to me than a fourth cousin.

~~~
awl130
unless you are thai yourself? are we supposed to presume that everyone is not-
thai on this site?

~~~
kbd
Fair enough. I'm not even Asian.

