The human orgasm, and particularly the female orgasm, is a mysterious thing. It's an area of scientific research where relatively few dare to tread, and when they do it can be hard to convince funders that the work is worth the money.
There are plenty of big questions left in sexology, but here are 10 of the top discoveries in the field from 2016.
10. Evolutionary clues to the origin of the female orgasm
Scientists said that they had found the evolutionary purpose of the female orgasm: to stimulate ovulation. In a paper published in the Journal of Experimental Zoology scientists looked at the female orgasm in other mammals. During orgasm, prolactin and oxytocin are released by nerve cells. These hormones also seem to have a role in stimulating ovulation.
The researchers argued that the female orgasm was once necessary to induce ovulation but humans have since evolved not to require orgasm in order to ovulate.
9. Communication matters: Bisexual women rate their top encounters of the past year
When asked to think back over the past year and recall their best sexual experiences, bisexual women rated those where they felt they had a good emotional connection to their partner. This was more important in how highly women thought of their encounters than the mechanics of what went on, according to a paper published in the Journal of Sex Research.
A sexual encounter was likely to be remembered fondly if study participants rated them highly in terms of emotional security, how well they got on after sex and their ability to communicate.
8. Feeling comfortable in a relationship gives women more orgasms
Women who frequently have orgasms don't manage it by more masturbation or experimenting with a greater number of partners during their lifetime, according to a study published in the journal Socioaffective Neuroscience and Psychology.
A series of national surveys in Finland found having more orgasms was related to women's sexual self-esteem, the importance they placed on having an orgasm, and how easily they could talk about sex and orgasms with their partners. It also helped if the initiation of sex was equal between partners, and if the woman's partner had good sexual techniques, the study found.
7. The clitoris vs vagina debate is missing the point
It doesn't make sense to argue about the relative qualities of clitoral and vaginal orgasms, scientists say. In a paper published in Socioaffective Neuroscience and Psychology, researchers say that such a complex and nuanced phenomenon can only be thought of as a whole, rather than as the sum of its parts.
"Stimulation of one or all of these triggering zones are integrated into a whole set of sensory inputs, movements, body positions, arousals and cues related to context," study author James Pfaus of Concordia University writes in the paper.
"That combination of sensory input is what reliably induces pleasure and orgasm during masturbation and intercourse."
6. Orgasms are an evolutionary currency
Orgasms can be thought of as an evolutionary currency, according to another paper published in the journal Socioaffective Neuroscience and Psychology. It's also a currency that can be used in relationships, writes study author Diana Fleischman of at the University of Portsmouth.
"The idea is that pleasure and bliss states are the currency with which evolution programmes us to do the things it wants us to do," Fleischman told IBTimes UK at the time. "We in turn can use that in turn to influence other people."
5. 14% of Midwestern students lose their virginity in a car
A survey of 706 students at a university in the Midwest of the US found that 60% of them had had sex in a car at least once – and 14% of them had lost their virginity while parked in a car.
The authors wrote in the study, published in the Journal of Sex Research: "These data, including personal stories of memorable incidents, revealed that despite discomfort, body bumps and risk of being caught, sex while parked was primarily a positive sexual and romantic experience for both men and women."
However, a small percentage of people – 2.5% of men and 4.3% of women – in the study reported being sexually coerced into sex in a car.
4. There is a 'Goldilocks zone' of sexual experience
People who have had a lot of sexual partners, or very few, are less attractive as potential partners than people in the middle, according to a study published in the Journal of Sex Research.
"In all but the most conservative societies, any two adults embarking on a relationship are likely to have a more or less extensive résumé of past romantic experiences: first loves, unrequited loves, old flames, drunken mistakes, and so on," the authors write in the paper.
There was a particularly dramatic drop-off in attractiveness with people who had had very many sexual partners. Contrary to popular wisdom, there was little difference between men and women in how attractive they were given their number of sexual partners.
3. Talking about sex helps keep libido high in long-term relationships
Long-term relationships are known to be tough for people's sex drive. Scientists did a large study on couples in long-term relationships and how well they were faring in terms of libido, publishing their research in the Journal of Sex Research.
The study looked at 38,747 people who had been in a relationship for at least three years. More than 80% of people reported being sexually satisfied in the first six months of their relationships. This figured dropped to about half for couples who were at least 3 years in.
The factors that were most closely linked with sexual satisfaction included receiving more oral sex, having more consistent orgasms, having a greater variety of sexual activities, taking the time to set the mood and making sure that the couple was communicating well about sex.
2. Oral sex on women is seen as 'a bigger deal' than oral sex on men
A survey of 16 to 18-year-olds in England found there was a narrative of "give and take" in terms of sex, but that more men were squeamish about going down on women than the other way around.
Men found it easier to refuse to give oral sex to a woman than women did to men, the survey found. Women reported anxiety about "taking too long" and about being the recipient of sexual attention, rather than having vaginal intercourse, which was seen as a more mutual activity.
The results tie in with previous studies that found that 43% of men expected to receive oral sex in a relationship, whereas only 20% of women felt the same entitlement.
1. Men fake orgasms too – in up to 1 in 4 encounters
There is little research on why people fake an orgasm, particularly men. A paper looking into men's reasons for feigning an orgasm found that they did it as often as 1 in every 4 times they had sex.
The study used an online survey of 230 young men, aged 18-29 years old, who had pretended to have an orgasm with a current sexual partner at least once. Men most frequently reported faking an orgasm during vaginal sex.
The reasons men feigned orgasm ranged from not wanting to hurt their partner's feelings or because they didn't really want to be having sex, the authors write in the study, published in the journal of Sex and Relationship Therapy.