Biology of Cross-Sex Friendships

“I have never met anyone who loves virtue as much as he loves sex.” – Confucius

We are attracted to our friends.

This attraction is not physical, but purely psychical. We befriend people who we connect with and want to share our lives with. Spending time with people homologous to us makes us feel accepted and understood.

At times, we share such emotional connections with members of the opposite sex. Platonic friendships between men and women are less stigmatized than ever before – they  blossom despite the raised eyebrows of conservative grandparents. In fact, having a social circle relished with both sexes broadens perspective, deepening understand on how distinctly men and women experience the world.

But perhaps our grandma has a reason for suspicion. Human biological instincts are yet to align with the growing trend of cross-sex friendships. The phenomenon intertwines with mate selection. Attempts to categorize friends as definite no-no’s with terms like ‘friendzone’ simply bottle sexual feelings, whereas arrangements like ‘friends with benefits’ allow desire to roam without commitment and security. Miscommunication and selfishness between friends can lead a valuable, heartfelt connection to a swift, unwanted end.

But what creates these complications? And what are we supposed to do when developing sexual attraction towards our best friend?

Friendship is a biological reaction


The hypothalamus releases chemicals that create trust towards friends. One such chemical is oxytocin. Oxytocin is a mediator of the serotonin release pathway, rewarding us with chemical candy for positive social behavior. It makes the brain happy.  These hormonal boosts increase tendency for altruistic behavior and reduce stress. In contrast, persistent loneliness has been defined as a source of anxiety and unhappiness,  and it can even be lethal. Evolution has programmed us to prefer social interactions, be it for cooperation or being part of a community.

Since we associate positive feelings towards specific people, we want to spend more time with them, get to know them, and share experiences with them. This time spent together is like leaving a pot to stir on medium heat, and sometimes it may overheat.

Friendly and sexy hormones are similar


The hormone cocktails for romantic and altruistic relationships closely resemble one another. This can lead to unwanted mixups and a truly emotional mess.

Both oxytocin and serotonin play a role in tipping a friendship into romance. The pleasurable serotonin (along with dopamine and adrenaline) is the first to create limerence – that starry-eyed zombie response felt during a strong crush. Oxytocin (and vasopressin) are then released as emotional glue later in a relationship. This promotes family making: oxytocin emotionally couples individuals during orgasms and is released at childbirth and lactation to bond a mother and child.

But what if you are not looking for a partner at all?

Technically, we all are. Both male and female brains are on a constant lookout for potential mates, using pheromones and appearance as yardsticks. We can pinpoint the gender and relative attractiveness of a stranger within fractions of a second. Again, evolution has got our backs. It ensures that we procreate only with the utmost viable members of the species.

Men and women differ in biological goals


A 2012 study showed that men are more likely to feel attraction in a cross-sex friendship than women. Women underestimated the level of attraction in a friendship, whereas men tended to overestimate. This difference in interpretation is due to the different biological goals dedicated to each sex. Women tend to be more cautious and picky as they are limited in the number of biological children. In contrast, men have less restriction in reproduction and have evolved to be opportunistic.

This may be the reason why a majority of males on a college campus see their female friends as possible love interests.

Communicating individual needs 


Sex is a complicating variable in a friendship. At best sex is a shared ecstatic experience, and at worst a destructive obstacle. If mishandled, sexual feelings can create a barrier of awkwardness, confusion, and resentment between friends. In fact, participants of a research study who reported no sexual attraction to their friend were in significantly longer friendships compared to those who felt an attraction.

The best way to deal with emotions is to communicate them. The other person may be more kind and understanding than you think. Maybe you both need a supportive and empathetic camaraderie? Someone to just study, flat-share or have fun with? Honest discussion of personal needs can clarify the goals of the friendship, and avoid hurt in the future. At the end of the day, a desire for romantic involvement can not be deployed. Being respectful of one’s existing partner, single status, or hesitation from a friend is a simple conscious choice. A friend may not see you as a potential mate due to their genetic and environmental background – the quicker you accept this, the quicker you can more on. In this sense, it’s really not you, but purely them.

Being aware of the pitfalls in cross-sex relationships can prepare us for the worst, as well as the best. Indeed, honest companionship and authenticity can lay the groundwork for long-term relationships – and the best marriages are built on friendship.

Watch Next


Matthew Hussey on individual cross-sex friendships –

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