Biological sex

Biological sex is the set of physical characteristics used to classify bodies as female or male: chromosomes, gonads (ovaries or testes), hormones and genital anatomy. In the vast majority of people all these layers line up. In a minority they don't fully fit the typical definitions: these are intersex variations.

What makes up biological sex

• Chromosomal sex: most commonly XX or XY, although other combinations exist, such as XXY or a single X chromosome.
• Gonadal sex: having ovaries, testes or, rarely, tissue of both.
• Hormonal sex: levels of oestrogen, progesterone and androgens such as testosterone. Everyone has all of these hormones, in different amounts.
• Anatomical sex: the internal and external genitals and the secondary sex characteristics that appear at puberty, such as body hair or breast development.

How it develops before birth

In the first weeks of pregnancy, the embryo has gonads that can become either ovaries or testes. If a gene called SRY, usually found on the Y chromosome, is present, it switches on the genes that form testes. These produce testosterone and other hormones that lead to male genitals. Without that signal, development follows the female pathway. A change at any of these steps can lead to a variation. To learn how sex cells form and how pregnancy begins, see human reproduction.

Biological sex, assigned sex and gender

• Biological sex: the physical characteristics of the body.
• Sex assigned at birth: the category recorded at birth, almost always based on the appearance of the external genitals.
• Gender identity: a person's inner sense of being a woman, a man or something else. See our entry on gender.

These are separate from sexual orientation, which is about attraction to other people.

Intersex variations

WHO describes intersex people as those born with sex characteristics that don't fit typical male or female bodies, and notes that these variations occur regularly in human biology. Medicine uses the term "differences of sex development" (DSD). A 2006 medical consensus proposed "disorders of sex development", but many intersex people reject it because it presents their bodies as a mistake. That is why "variations of sex characteristics" is also used today. The word "hermaphrodite" is considered inaccurate and stigmatising.

Some examples:

• Klinefelter syndrome (XXY): affects about 1 in 650 newborn boys. Many are never diagnosed.
• Congenital adrenal hyperplasia: the classic form affects about 1 in 15,000 births. Some forms need treatment from the first days of life.
• Complete androgen insensitivity: about 1 in 100,000 births with XY chromosomes. The body doesn't respond to testosterone and develops a female appearance.

How many people are intersex?

There is no exact figure, because it depends on which variations are counted. Italy's National Institute of Health (Istituto Superiore di Sanità) notes that the scientific literature gives figures between 0.018% and 1.7%. The higher figure, used by the UN, includes variations that don't affect the genitals. Visible genital differences at birth occur in about 1 in 4,500 births.

Rights and healthcare

Being intersex is not an illness in itself. Some variations need medical follow-up and others don't. The UN human rights office calls for a ban on medically unnecessary surgery and procedures on intersex children. These are often carried out without the child's consent and can cause infertility, pain and lasting distress. Where health allows, the recommended approach is to wait until the person can decide for themselves. Families need clear information and specialist teams. For more background, see our guide to anatomical and gender diversity.

Common myths

• "Sex is just about genitals": it involves several biological layers.
• "There's nothing beyond XX and XY": other combinations exist, and some, such as XXY, aren't rare.
• "Intersex people always know from birth": many variations are found at puberty, when trying for a baby, or never.
• "Sex and gender are the same": they are linked but describe different things.

Why it matters for health

Understanding biological sex well helps in medicine. Some medicines work differently depending on physiology, and some screening depends on the organs a person has. For example, cervical screening is for anyone with a cervix and prostate checks for anyone with a prostate, whatever their identity. That is why health professionals should ask about anatomy respectfully, without making assumptions.