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How Many Friends Does One Person Need?

Dunbar's Number and Other Evolutionary Quirks

Robin Dunbar

We live in massive cities with tens of millions of inhabitants, which shows our cultural flexibility. Yet the number of people that we know personally is still no more than 150. And it is 150 because our minds lack the capacity to cope with any more.

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It's not just a matter of remembering who is who, or how they are related, but rather how can I use my knowledge of the individuals involved to manage these relationships when I need to call on them

We have an innermost group of 3 - 5 people. These are ones you call on in times of trouble. Above this is a group of 10 -15, the 'sympathy' group: those whose deaths wd leave you distraught. The next level is 30 - 50 people; ones you'd make sure to catch up with at least once a year, and finally the whole 150 which includes people who you don't care if you see them or not.

We care abt kinship. Canadian researchers pulled a list of uncommon names off the US Census then emailed hotmail address combinations to see who wd take the trouble to reply. If recipient shared neither first or last name you got 2% response, shared first nmes 4%, shared surnames 6% and both names in common 12%. If the names were very rare they got even larger and longer response, asking abt family background

Charles Darwin didn't invent the theory of evolution. It dated back for at least a century to thinkers such as Cuvier, Buffon, Lamarck and Charles' grandfather Erasmus Darwin. But they were locked into a medieval midset where a God planned Creation steadily moved mankind closer to the angels. Charles Darwin's contribution was to recognise that natural selection is the engine that drives evolution. This provide d a mechanism that didn't need a creator, and it explained how individual traits enabled individual animals to reproduce more successfully.

Darwin's ideas have been expanded and modified, but everything still based on his elegantly simple idea: organisms behave in ways that improve their chances of getting their genes into future generations.

Monogamy: do you find relationships hard work? Do you find yourself struggling yet again with your partner's foibles? If so, yr in good company. In birds and mammals, those which mate monogamously have the biggest brains relative to their body size. Those that live in large anonymous flocks or herds and mate promiscuously have much smaller brains.

This becomes even more obvious when look at the two kinds of birds; those like the garden robins and tits, who choose a new mate every breeding season, and the birds of prey, owls and most crows and parrots that mate for life. It is this second group which have the biggest brains of all the birds.

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It's impt because brain tissue is so expensive to grow and maintain - only yr heart liver and guts are more expensive. So what makes monogamous pair bonding so cognitively demanding? Lifelong monogamy carries huge risks - if you choose a bad mate, a lazy or sick one, you jeopardise yr chances of passing on yr genes. And, you have to be able to co-ordinate yr behaviour with yr mate. Unless you can both feed, your eggs will not be hatched, and you need to eat yr body weight in prey each day to stay alive. So next time you struggle to figure ouy why yr spouse has behaved so badly yet again, console yourself with the thought that evolution has equipped you with the means to figure out how to make the best of a bad situation.

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Interesting genetic pattern in souther England today. About half the inhabitants carry the Y chromosomes of continental Anglo-Saxons, dating back to the invasions of the 7th 8th and 9th centuries of the Angles Saxons and Jutes. But this doesn't apply to the female genes. It looks like a relatively small number of Anglo-Saxon men had much more than their share of the Celtic women, to the exclusion of the Celtic men.

And though the Scots didn't have same trouble with Romans and Saxons, they weren't completely safe. Analysis of Icelandic genes shows that while the male Y chromosomes come from Scandinavian stock as you'd expect, 50% of the female genes are of Celtic origin. They come from Scotland and Ireland - a convenient spot to drop off on en route to a new life in Iceland, especially if your own womenfolk weren't particularly keen on the prospect of a gruelling sea voyage then a hard life on a volcanic outcrop in middle of the North Atlantic.

In mammals the length of gestation is dictated by size of the brain. It seems that brain tissue can only be grown at a set rate, so you can only grow yr brain bigger by growing it longer. On that basis, human gestation shd have a gestation of 21 months. Several million years before we started to get big brains, our ancestors found it was an advantage to walk upright. This led to the evolution of our bowl-shaped pelvis which is a much better base to balance trunk and head, giving capacity for standing walk and ability to move over big distances, either for hunting or migration. But this has trade-offs, one of them being a weak lower back. A flexible spine has many advantages in throwing spears or cricket balls. When humans started to grow bigger brains, the existing pelvis allowed few options, one of which was to give birth early, to a baby only part developed. Whereas other mammal babies are active and busy within hours of birth, human babies take a full year - the missing 12 months of their gestation in utero.

For that year, babies basically just lumps of flesh that need lots of TLC. And since TLC is hard work for parents, babies better have lots of winning ways and appeal. And a woman needs a man by her side to help. But if the baby is not his, it may cause problems. So should baby look just like dad, or shd it look like all other babies?

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The 'Race to be male': Basic bio lesson that we get an X chromosome from mum and either an X or a Y from dad. XX makes you a girl, and XY makes you a boy. But this is only part of the story. You might have an XY pair and still not turn out a boy. In fact, you only get to be male if a whole lot of things fall into place at the right time. Only if the foetus lays down the right density of fat cells early on does testosterone get released to convert the default female embryo to male. But there can also be a huge range of possible combos - XXY, XXYY, XXXYY and XYY. In turtles and crocodiles, your sex depends on the temperature of the nest - warm ones produce males, cooler ones females (but the reverse in turtles)

A common belief among IDers is that evolution depends on blind chance - mutations randomly producing small changes that gradually add together. Hence their claim that natural selection is the equivalent to asserting that a whirlwind could assemble a jumbo jet by blowing through a junkyard. But they miss the basic point of Darwin's contribution - the mutations may be random, but it is natural selection that chooses the few effective ones.

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2005 was a worse than average year - some 400,000 people were killed by natural disasters: about 5 times the normal average. But in same year a million people were killed on the roads, and 8 million children died of preventable childhood diseases

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We are supposed to be rational, but we make many decisions about people based simply on their appearance. Salvadore Dali understood this - even as a penniless young painter he lived a lifestyle of flamboyant and conspicuous opulence way beyond his means. Everyone thought he was doing extremely well because he was obviously attracting many wealthy clients, so all the wealthy clients came to him to commission paintings.

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Could have predicted Obama wd win 2008, because he was taller than John McCain and had a more symmetrical face. Since 1900 the taller candidate has won 3x number US pres elections. Abraham Lincoln had a hugely unsymmetrical face (he'd been kicked there by a horse as a child) but he was 6 feet 4 against Stephen Douglas 5 feet 4

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Politics is all about emotion - study of how people reacted to scary pictures or loud noises, as measured by pulse rate and sweat response. The more conservative you were (pro-high mil spending, death penalty, literal truth of bible anti- premarital sex, immigration, gun control or abortion) the higher yr panic response. The more extreme yr political position partic on right, more likely you were to respond to emotional appeals from demagogues rather than stop and think rationally.(study by Douglas Johnson Uni of Nebraska)

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Problem with juries: they are not 12 independently minded people trying to evaluate the facts - most are juries of 1 or 2 people. One or two strong willed people always dominate - we have evolved in societies where its best if everyone does the same thing, with a few good leaders and a lot of sheep. Dunbar suggests solution is to hire professional juries - people educated and qualified enough to understand the complexities of modern forensic science and complex arguments.

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"An education is not just a technical training in the arcane knowledge of a discipline, whether that be history or science. It is training in how to think and evaluate, how to marshal evidence for and against a position, how to approach a problem critically without falling prey to prejudice and preconception. Those are skills that everyone from bank manager to politician, journalist to local govt functionary, needs every day." (214)

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People who are neither famous nor wealthy still have to find a mate. How? Evo theory suggests shd lower expectations and make most of what you can get. Straight out of Jane Austen. When dating sites analysed, realism obvious: no point investing resources trying to date someone too far above you on the social scale. We learn early how we stand in the mating market, and adjust our aspirations accordingly. We might yearn for a film star but it only takes a couple of cold shoulders for reality to intervene. And so, like tends to settle for like.

One striking feature of traits women seek is emphasis on pairbonding and family, signalled by words such as loving, warm, family-minded, gentle and dependable. 45% of women wanted at least one of those traits in a prospective partner, but only 20% of men used any of those words to describe themselves.

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This probably reflects a cultural lag - in the past, financial security a primary concern for women, but many more women now self-sufficent and less dependent on men to provide. But men still haven't woken up to this, and are still pushing trad traits of manliness and wealth.

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Study of even supposedly monogamous birds and animals shows it is not fixed and immutable instinct - not hard-wired into the brain. Couple of examples - marmoset and Tamarin monkey. Both are normally monogamous, with the males usually raising the young. But when there is an excess of males, you get roving polygamy, with males having a succession of female partners. What happens is, a lower status male who can't get a partner becomes a 'helper-at-the-nest', assisting in rearing offspring that aren't his own. This leaves the father free to find another female, knowing that first brood will get cared for. The second male gets his turn when the original female comes back into season. The females don't seem to care as long as someone is looking after the kids. The high status males get extra chances to mate, the females fare no better or worse, and the low-status males make the best of a bad situation.

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Birds seem to be monogamous - 90% of species pair up for the season at least. But DNA fingerprinting showed that at least 20% of eggs had different sire. Males don't know if it's their own offspring they are raising, but if they abandon partner, she will be unable to raise chicks on her own and they will die. Suggest it actually makes sense for a man to not enquire too closely - if he treated all partner's children as his own, as long as that allows him to maintain a good relationship with her and thereby gain access to most of her future reproduction. For some men. rearing a few offspring sired by another may simply be the cost some males have to pay in order to reproduce at all.

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Male motivation for extra sex on the side is obvious, but what's in it for the female? Two possibilities. First, she wants mate with best genes and best (richest/best hunter etc) provider. But few males come with high ratings on all dimensions, and those who do are usually swamped by suitors. So best option is to team up with a good provider and allow him most conceptions, while giving the rest to better quality mates as and when she can. Second, it's a way of keeping partner attentive. (But it is a fine line to tread - all over the world, the vast majority of spousal murders are triggered by actual or suspected infidelity).

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Why are humans religious? In our everyday lives we make at least some effort to check the truth of claims. Yet when it comes to religion, it seems we are most persuaded by stories which contradict most of laws of physics. We like tales of supernatural beings walking on water etc, but at the same time we expect them to have normal human feelings and emotions.

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Given that religious behaviour seems to be universal among humans and often very costly, and seems to contradict basic 'selfish-gene' biology, why is it so ubiquitous? At least 4 ways beneficial: First, it gives us an explanation of the world, and from that, a chance to control it (by asking spirits to intercede on our behalf). Second makes us feel better about life (Marx's "opiate of the people"). Third, religious groups are able to impose moral codes, and so keep social order. And finally, gives people a sense of community.

The real benefits of religion seem to be in creating close-knit communities. It is only when religion gets taken over by the state an becomes large scale that problems arise. It seems that the psychological forces it can call upon are so powerful as to be able to turn rational people into bigoted mobs. It is these psychological mechanisms that have been exploited down the ages by political elites to subjugate the rest of the community.

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