Thirty-five years ago, millions of people around the world suddenly became "healthy." It was on that day — May 17, 1990 — that the World Health Organization (WHO) removed homosexuality from a list of human diseases.

Until then, same-sex love was considered as a kind of mental illness. Those affected were often locked up in sanatoriums or prisons and "treated" with electric shock therapy and other questionable psychotherapies.

But homosexual, bisexual and transsexual people are not — and never were — sick, said Klaus M. Beier, the director of the Institute for Sexology and Sexual Medicine at the Berlin Charité hospital. 

Human sexuality is characterized by its diversity, Beier said.

"These days it's clear that no one chooses their sexual orientation. It's a matter of fate, not choice. Sexual orientation — or what experts call a 'sexual preference structure' — develops during puberty, influenced by a person's sex hormones. Their sexuality is determined in adolescence — what they find physically attractive and the kind of sexual interactions they desire."

After this developmental phase in adolescence, the respective sexual preference remains stable, according to Beier. 

"It arises in adolescence and is then stable for life, despite the desire in some people for sexual orientation to change, for example, if there's social pressure to be like everyone else," said Beier.

Non-heterosexuality made a problem in many places

Universal human rights include the right to free sexual orientation. Sexuality is, and has always been, diverse. It is neither a fad, nor is it limited to particularly liberal societies.

"According to the data we have, same-sex orientation accounts for about 3-5% of a population, and that applies across cultures. Human sexuality […] is characterized by this diversity — and cannot be had any other way," said Beier. 

It follows that it's wrong to judge or even condemn someone because of their sexual orientation.

Nevertheless, the sexual orientation of individuals polarizes entire societies. In some cases, this leads to their exclusion, discrimination and persecution. Homosexuality, for example, is punishable in at least 67 countries, and in seven countries there is even a threat of death for same-sex sexual acts.

How does sexual orientation develop?

How sexual orientation develops is a simple question to which there is no simple or conclusive answer. There is no one cause for sexual orientation. But researchers have attempted to understand how sexual orientation develops through analyses of genes, hormones and sociocultural factors.

"The current thinking is that there are a number of factors. So far, no one has been able to find any single factor that could be named as the cause of one person being same-sex oriented and another being opposite-sex oriented," said Beier.

It can be assumed that a complex interplay of biological and social factors is responsible for the development of sexual orientation.

The biological factors influencing the development of sexual orientation include a person's genetic makeup, as well as (prenatal) hormones and chemical substances.

But sexual orientation is not hereditary. Family and so-called twin studies suggest homosexuality can be common in any one family. However, the genetic markers found in these studies were not deemed to be significant enough for researchers to conclude there was "homosexuality gene."

How do hormones and social influences affect sexuality? 

Hormones, such as testosterone, and chemicals in the body, such as pheromones, may also be partly responsible for the development of sexual orientation. Pheromones are chemical secretions that influence sexual behavior.

Studies show that male pheromones stimulate hypothalamus activity in both heterosexual women and homosexual men — but not in heterosexual men. The hypothalamus is a gland in the diencephalon that influences our instinctive behavior and sexual functions.

Dolls and clothes for girls? Tools and cars for boys? Toys that are typically seen as being for females or males have no influence on sexual orientation. The same applies to education.

It's true that some people do not live out their actual sexual orientation until later in life. That said, our sexual preference does not change as we age.

"We have very strong indications that this is not possible," said Beier. "There are followup studies on sexual orientation. There were these regrettable 'conversion attempts' in same-sex oriented men. This was attempted in a larger study in the United States in the 1970s, without any success. It's strong evidence that sexual orientation is very stable."

While sexual orientation develops during puberty, the development of gender identity begins in childhood and is completed in most people by the age of 5 to 6 years, said Beier. From this age on, children can "see themselves in their sexuality in the future and thus make assumptions about their future as a man or woman," he said.

No sexual orientation is a disease

Once sexual orientation has been established, it never changes. Not even through "seduction" or through early sexual contacts. "There is nothing to it," said Beier. "An important proof of this: There are many people who have had same-sex sexual contacts in their youth, but who are not same-sex oriented."

The decisive factor for identity development is whether children receive support or rejection from their parents. If children and adolescents encounter strong rejection, they often develop weaker self-esteem by comparison. If parental rejection is associated with a child's sexual identity or orientation, it can lead to depression and suicidal thoughts.

Especially in societies where sexual minorities are excluded and persecuted, an unprejudiced debate about sexual diversity is very important, said Beier.

From a scientific point of view, no sexual orientation is a disease or "unnatural." What is tolerated, or what is considered "normal" or "unnatural," is determined by social norms. These norms can change greatly depending on time and context. But human nature does not change.

This article was originally written in German.

Correction: A previous version of this article stated that Sharia law mandated the death penalty in Mauritania, Nigeria, Somalia and South Sudan. Sharia law is only applied in 14 of Nigeria's northern states, not the whole country. South Sudan is a secular state and does not practise Sharia law. While homosexuality is banned, the punishment is not death, but up to 14 years in prison. (27.05.25)