5 Truths About the Biology of Cross-Sex Friendships

Laura Turpeinen
4 min readDec 26, 2019


“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 psychological. We befriend people who feel familiar, who we connect with and want to share our lives with. It makes us feel accepted and understood.

Sometimes, we share such emotional connections with members of the opposite sex.

Platonic friendships between men and women are less stigmatised than ever before — they blossom despite the raised eyebrows of conservative grandparents. In fact, having a social circle relished with a range of sexes broadens perspective and deepens our understand on how distinctly individuals experience the world.

But perhaps previous stiff-minded generations have reason for suspicion. Human biological instincts are yet to align with the growing trend of cross-sex (or same-sex homosexual) friendships. For cross-sex friendships, the phenomenon is only a few pints away from intertwining with mate selection.

Understandably, we do make a conscious effort to battle such messy mixups, but often through messed up strategies:

Attempts to categorise friends as definite no-no’s with terms like friend zone simply bottle up sexual feelings and create an ugly grudge that reveals itself after a friendly bottle or two.

Similarly, arrangements like friends with benefits is by definition more than a platonic friendship, allowing desires to roam without clear commitment into a sustainable relationship. Miscommunication and selfishness between honest friends can lead a valuable, heartfelt connection to a swift, unwanted disaster.

What then creates these complications?

And more importantly, what can we do while developing a sexual attraction towards our very best friend?

Friendship is a biological reaction

The small region of the human brain called the hypothalamus releases chemicals that create feelings towards friends.

One such chemical is oxytocin. Oxytocin is a mediator of the serotonin release pathway, rewarding us with chemical candy for positive social behaviour. Oxytocin makes the brain happy. These hormonal boosts increase tendency for altruistic behaviour and reduce stress. In contrast, persistent loneliness has been defined as a source of anxiety and unhappiness and it can even be lethal. We are programmed to prefer social interactions, be it for cooperation or for simply belonging into a community.

Since we associate positive feelings towards specific people, we want to spend more time with them, get to know them, share cool experiences with them. This time spent together is like leaving a pot to stir on medium heat, sometimes resulting in a hotter brew.

Friendly and sexy hormones are similar

The inconvenient truth is that the hormone cocktails for romantic and altruistic relationships closely resemble one another.

Both oxytocin and serotonin play a role in sailing a friendship into a frenzy. The pleasurable serotonin (along with dopamine and adrenaline) is the first to create limerence — the starry-eyed zombie response felt during a fiery crush.

Oxytocin (and vasopressin) are then released as emotional glue later in the relationship to promote family planning. Incidentally, oxytocin couples individuals emotionally during orgasms, and is also released during 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 wired into an albeit variable lookout for potential mates, using pheromones and appearance as yardsticks. As an example, humans are great at pinpointing the gender and relative attractiveness of a stranger within fractions of a second. This ensures that we procreate only with the utmost viable members of the species.

Men and women differ in biological goals

Sexist, I know.

A 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 female friends as possible love interests, whereas girls laugh off certain close candidates.

Individuals need to communicate their needs

Sex is a complicating variable in a friendship. At its best, sex is a shared ecstatic experience, and at its worst a destructive obstacle. If mishandled, sexual feelings can create a barrier of awkwardness, confusion, and resentment between friends.

In fact, leaving sex out from a friendship altogether could be the smartest thing to do: participants in a study who reported no sexual attraction towards their friends were in significantly longer friendships compared to those who felt an attraction.

The best way to deal with rising emotions is to communicate your personal needs.

No matter how scary, the key is to keep the other party informed — your friend may be kinder and more 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 clarifies the goals of the friendship and avoids hurt in the future.

At the end of the day, a desire for romantic involvement doesn’t have to be deployed: enjoying the friendship as it is, to rather be single or respecting someone’s existing partner are great reasons to not sleep with a friend. The quicker you understand, accept and communicate your own needs, the quicker you can move on, relax and focus on something else in life.

That said, often the happiest marriages result from a well communicated friendship.

Make of that what you will.