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A weekly show where knowledge junkies get their fix of trivia-tastic information. This week, John discusses the science of friendship!

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Hi, I'm John Green.

Welcome to my salon. This is Mental Floss video and did you know that we lose half of our friends every seven years?

I mean, on average. One Dutch sociologist studied about a thousand people and their friendships and it turns out that every seven years, half of the people in our close social group disappear, meaning there's constant turnover when it comes to friendships. And that's the first of many facts about friendship and loneliness that I'm gonna share with you in this video today.  Our number of friends remains pretty consistent.

A 2014 study in the proceedings of the National Academy of Sciences had 24 student participants who were on the verge of moving from high school to college. Half were men and half were women. The researchers gave them all phones and then sent them three questionnaires about their friendships over the course of 18 months.

They saw a lot of turnaround when it came to individual friends, but the number of friends, at least that the participants called often and considered close, remained consistent.  For me, the number has always been one. I have one friend. I don't really find that I need another one.  One recent survey actually attempted to total the number of friendships a person has in a lifetime.

The researchers determined that British people have an average of 396 friends over the course of their lives. But that's a fairly new phenomenon. I mean, people generations ago wouldn't reach that number because they didn't move around as much as we have or have the technology that we do, which reminds me- I do actually have more than one friend.

It's just that most of them are on the Internet.  One study from the Department of Rehabilitation and Aged Care at Flinders University found that people with large networks of friends actually live longer. Good news for me if we're talking about number of Facebook friends, but otherwise I'm a bit concerned. Anyway, the researchers surveyed 1500 older people for 10 years and they found that people with a bunch of friends lived 22% longer than people with fewer friends.

Also one meta-analysis published in the scientific journal Public Library of Science Medicine found that low social interaction has similar impacts on our health as smoking 15 cigarettes a day or having more than six drinks a day. Fortunately, if you do all three of those things together, you're fine, according to a survey I just made up.

Having friends might also help us recover more quickly from disease. At Ohio State University, a group of researchers studied 200 women who had survived breast cancer, and it turned out that the women with strong groups of family and friends actually experienced less pain, depression, and fatigue than women who described themselves as lonelier.  And based on antibody levels, the researchers found that women with more friends might even have had stronger immune systems.

In 2012 there was a study that looked at cortisol levels of 103 students in fifth and sixth grade while they recalled what happened 20 minutes prior.  As you'll remember from previous Mental Floss videos, cortisol levels are a good indicator of stress. If a student was with their best friend during any negative experiences, their cortisol levels didn't increase, but they did increase when they were alone.

But not all friends are good for you, there are actually health risks associated with love-hate friends, also known as ambivalent friends, or frenemies. That's a Gossip Girl reference in case you didn't catch it, I'm hip, I'm with it, I'm contemporary, I'm quoting television shows that went off the air four years ago.

So according to a survey from the University of Utah, half of our friend groups are made up of those people, which seems a bit high, but we'll go with it. Anyway, the researchers hooked people up to blood pressure monitors for a couple days, and it turned out that when people spent time with their ambivalent friends, their blood pressure rose. In fact, their blood pressure actually rose higher when with frenemies than with people they didn't like at all.

People who are able to name three close friends who are also co-workers are actually 96% more likely to report extreme satisfaction with their life. BUT, workplace friendships might also be distractions that decrease productivity, which is why I have banned them. You're not here to be extremely satisfied with your life, people. You're here to make video.

A 2007 study published in the University of Aukland Business Review reported that 445 workers had been surveyed and over 200 were able to name examples of a distracting friendship with a colleague.

And speaking of the workplace, according to LinkedIn, 68% of 18 to 24 year olds admit that they would get rid of a friend if it meant they could get a promotion.

Jumping back in time to preteen friendships, a 2015 study published in the journal Psycological Science attempted to find out why 7th grade friendships end by enlisting 410 middle schoolers and asking them questions about their friendships every year up until their senior year of high school. By that time, only 1% of 7th grade friendships remained. The researchers found that friendships were most likely to end for reasons like differing grades, genders, and popularity levels. I've actually be able to maintain 100% of my friendships from 7th grade, but only because 100% of zero is zero.

Speaking of high school friendships, research shows that peer pressure is effective. But not just for stuff like drug use and drinking, although it is effective for that. It turns out that teenagers who become friends with classmates who strive for good grades actually experience an increase in their own grades.

Also there was an interesting study about teenage friendship from the Max Planck Institute for Human Development in Germany and the Chinese Academy of Sciences. They collaborated to pose a scenario to Chinese and Icelandic children at 4 different ages between 7 and 15. In the scenario they'd already promised a good friend to help them talk out some personal issues, but then they got invited to an all-expenses-paid concert with a new classmate. Younger children from both cultures were most likely to choose the concert rather than their commitment to their friend, but by 15, kids of both cultures preferred to help the friend. So that's encouraging.

And then there's a 2015 study from the University of Sussex which found that kids who came from low-income neighborhoods were more likely to grow up emotionally resilient if they had a childhood best friend.

Another good time to make friends: college, which was actually the last time I was able to make a friend. I have no idea how to make friends in adulthood. Researchers from Purdue University spent 19 years studying the friendships of college best friends from the 1980s and they found that pairs who were very close in college, or had similar interests, were most likely to remain friends.

Being socially aware also makes you a better friend. Like, a 2011 study published in Psychological Science had college students participate with a friend and they'd separately get a list of personality traits and then rank them according to how much they disliked them and how much they thought their friend disliked them. Examples included skepticism, perfectionism, and obliviousness. And those who knew more about their friend's pet peeves were more likely to be considered a close friend.

And, unsurprisingly, it's been discovered that a good way to bond is to ask deep questions. Like, one study was able to get pairs of strangers to report increased closeness by having them ask each other increasingly personal questions.

And, of course, friendship also affects how we experience empathy. Like, in 2013 researchers connected 22 participants to an fMRI machine and then they mentioned delivering shocks to the participant, a friend, and a stranger. Brain activity was very similar when the person felt they were threatened and when it was their friend in danger, but different when it was the stranger.

A 2009 study had participants list and rank their 10 closest friends, and then they were asked to give points to those friends adding up to 100. Participants who were told that the scores would be revealed publicly gave each friend 10 points, but those who were told that the scores would remain private gave their best friend the most points and continued that pattern down the list. Yet another advantage to just having the one friend.

And friends tend to have similar genetic makeups. Like, one study showed that people with DRD2, a gene associated with alcoholism, are often friends, and people without DRD2 typically befriend people who also don't have it.

Similarly, lonely people have genetic makeups that resemble other lonely people. A study from UCLA found that people tend to be lonelier if they have plasmacytoid dendritic cells and monocytes in their white blood cells, which is probably a symptom of their loneliness.

And you knew we couldn't get through this episode without asking if heterosexual men and women can really be friends. Well, a 2012 study found 88 pairs of male-female friends who were in college. There were many confidentiality agreements so that they knew that the other wouldn't know what they were saying, and then, the friends were separated and interviewed about their attraction to their friend on a scale from 1 to 9. The mean number that men gave was 4.94, and women reported a mean of 3.97.

Male and female friends are also different. There's a lot of  stereotypes about this kind of thing, but one study from 2012 interviewed 269 students, some male and some female, to try to get to the bottom of it, and some interesting findings were, that for
instance women had higher expectations for friendships when it came to trust and intimacy, and men disliked getting a greeting kiss from their friends of the same sex more than women. I like greeting kisses, I think they're underrated.

According to the Pew Research Center 57% of US teens have met a
new friend online, though only twenty percent of teenagers have actually met an online friend IRL.

And finally I returned to my salon to tell you about the friendship paradox. It describes this bizarre phenomenon in which most of us
have fewer friends than our friends have. Sociologist Scott L Feld was the first to discover this and write about it in 1991. It's basically because the people with the most friends are the people most likely to be counted as quote "friends".

Basically friendship may make you healthier and happier and live
longer, but it's a giant pyramid scheme that's why I just got the one! Thanks for watching Mental Floss video, which is made with the help of all of these nice people. Tell me about your best friend in
comments so I can steal them, and as we say in my hometown, don't forget to be awesome.