Friendship: it means something different to all of us, and we probably all have our own definitions of the word. But did you know that friendship has its roots buried firmly in science?

There have been many studies into how and why we make friends, and the results tend to point towards scientific reasons.

So, because we’re all friends here, here are seven fascinating scientific facts about friendship:

1. There are scientific criteria for friendship

What’s the difference between a true friend and just an acquaintance? Believe it or not, science has the answer.
Behavioural scientists and psychologists describe a close friendship as a relationship that involves a long-lasting bond of sacrifice and shared moments. A true friend prioritises your happiness, never asks you to put your friendship before your principles, and you miss them when they’re gone.

2. Friendship is one of the very first things we learn about

In January 2014, the Journal of Experimental Psychology reported on research suggesting that babies as young as nine months old recognise that friends tend to have similar interests. Even before they can talk, it seems that babies still have expectations about the social world.

Says Amanda Woodward, a psychology professor at The University of Chicago: “Nine-month-old infants are paying attention to other people’s relationships.”
“Infants are able to watch two strangers interact […] and make inferences about whether those two people are likely to be friends.”
Even at this young age, we know what it takes to be friends.

3. Animals can make friends, too

Studies have shown that animals can form lifelong friendships with other animals – even if they’re not from the same species.
So far, the research has focused on chimpanzees, baboons, horses, hyenas, elephants, bats and dolphins – but there’s no reason to think that friendship is exclusive to these species.

Friendship offers a number of benefits, including reduced stress and superior health.
As long as the relationship is, like all friendships, mutually beneficial, then perhaps friendship can exist between any animals, regardless of species.

4. Our friends truly bring out the best in us

In 2013, research at the University of California, San Diego found that people look more attractive in a group than they do individually.
It’s a phenomenon called hierarchical encoding, but it might be more commonly known as ‘the cheerleader effect’.
Why does this happen? The study proposes that “the visual system automatically computes ensemble representations of faces presented in a group.”
Essentially, the more attractive people in a group serve to ‘raise the average’, so to speak, which makes the ‘average’ faces appear more attractive overall.

5. There’s a reason why childhood friendships end

It might have seemed like the friends you make in primary school will be your best friends for life, but how many of them do you still see now? Indeed, how many of them had you shed already by the time you reached secondary school?
A long term study by Florida Atlantic University (FAU) recently demonstrated that early teenage friendships are almost always broken apart by dissimilarity.
According to FAU professor Brett Laursen, “It causes conflict, it interferes with cooperative activities and shared pleasures and it create circumstances where one friend bears more costs, such as the friend who is less aggressive; or gets more benefits, such as the friend who has lower social status than the other. Dissimilarity disrupts relationship bonds.”
Dissimilarity may become less important as we get older – opposites attract, after all. But in those critical early years, compatibility seems to hinge on overall similarity, as opposed to specific traits.

6. We have more friends now than ever before

A 1993 primate study at the University of Oxford indicated that each individual is only capable of maintaining a certain number of social relationships at any given time. Anthropologist Robin Dunbar, who conducted the study, extrapolated this to humans, estimating the human limit for simultaneous friendships at around 150 – known as ‘Dunbar’s number’.
However, Dunbar has since reasoned that the rise of such social networks as Facebook may have raised the limit through increasing our memory capacity.

Nevertheless, those who maintain hundreds of friendships may do so at the expense of their closest relationships – those you turn to when you really need them.
So even though it’s now possible to have more friends than ever before, the fact remains that, when it comes to friendship, less is certainly more.

7. Love loses you friends

Finally, and sticking with Dunbar, the anthropologist also studied the effect that love can have on friendships.
Research has revealed that, when you gain a new romantic partner, you may lose two of your closest friends.
There’s a simple reason for this. It takes time to make a relationship work and the time you invest in your new lover is time that isn’t being spent with your old friends. On average, a new partner pushes out two close friends, leaving you with a decidedly smaller inner circle.
Dunbar calculated that the average person has five people to turn to in a crisis – their closest friends. However, those who are in a relationship only have four people – including their partner.
The study demonstrated that men and women are equally likely to lose a close friend when entering a new relationship. “If you don’t see people,” says Dunbar, “your emotional engagement with them drops off and does so quickly.”
Keep your friends close, folks – they need you and you need them!