[11] It is named after American endocrinologist Harry Klinefelter, who identified the condition in the 1940s, along with his colleagues at Massachusetts General Hospital.

In other cases, symptoms are more prominent and may include weaker muscles, greater height, poor motor coordination, less body hair, gynecomastia (breast growth), and low libido.

As teens, males with XXY may develop breast tissue, have weaker bones, and a lower energy level than others.

[19][20] By adulthood, individuals with KS tend to become taller than average, with proportionally longer arms and legs, less-muscular bodies, more belly fat, wider hips, narrower shoulders.

Additional abnormalities may include impaired attention, reduced organizational and planning abilities, deficiencies in judgment (often presented as a tendency to interpret non-threatening stimuli as threatening), and dysfunctional decision processing.

[19][25] Compared to individuals with a normal number of chromosomes, males affected by Klinefelter syndrome may display behavioral differences.

These are phenotypically displayed as higher levels of anxiety and depression, mood dysregulation, impaired social skills, emotional immaturity during childhood, and low frustration tolerance.

It demonstrated that males with XXY tend to be quiet, shy and undemanding; they are less self-confident, less active, and more helpful and obedient than other children their age.

They show a lower sex drive and low self-esteem, in most cases due to their feminine physical characteristics.

[4][25] Those with XXY are more likely than others to have certain health problems, such as autoimmune disorders, breast cancer, venous thromboembolic disease, and osteoporosis.

These patients are also more prone to develop cardiovascular disease due to the predominance of metabolic abnormalities such as dyslipidemia and type 2 diabetes.

This XXY chromosome arrangement is one of the most common genetic variations from the XY karyotype, occurring in approximately one in 500 live male births.

[41] Approximately 15–20%[42] of males with KS may have a mosaic 47,XXY/46,XY constitutional karyotype and varying degrees of spermatogenic failure.

Often, symptoms are milder in mosaic cases, with regular male secondary sex characteristics and testicular volume even falling within typical adult ranges.

Physical characteristics of a Klinefelter syndrome can be tall stature, low body hair, and occasionally an enlargement of the breast.

[4] The lifespan of individuals with Klinefelter syndrome appears to be reduced by around 2.1 years compared to the general male population.

[55] By 2010, over 100 successful pregnancies have been reported using in vitro fertilization technology with surgically removed sperm material from men with KS.

[60] The first published report of a man with a 47,XXY karyotype was by Patricia Jacobs and John Strong at Western General Hospital in Edinburgh, Scotland, in 1959.

Jacobs described her discovery of this first reported human or mammalian chromosome aneuploidy in her 1981 William Allan Memorial Award address.

In August 2022, a team of scientists published a study of a skeleton found in Bragança, north-eastern Portugal, of a man who died around 1000 AD and was discovered by their investigations to have a 47,XXY karyotype.

[62] In 2021, bioarchaeological investigation of the individual buried with the Suontaka sword, previously assumed to be a woman, concluded that person "whose gender identity may well have been non-binary", had Klinefelter syndrome.

[63] In many societies, the symptoms of Klinefelter syndrome have contributed to significant social stigma, particularly due to infertility and gynecomastia.

[citation needed] However, in recent years, increased awareness and advocacy have led to a reduction in stigma, with individuals diagnosed with KS more likely to receive proper medical care and support.

Advocacy organizations, such as the American Association for Klinefelter Syndrome Information and Support (AAKSIS), have played a crucial role in promoting understanding and improving the quality of life for affected individuals.

[64] This syndrome, evenly distributed in all ethnic groups, has a prevalence of approximately four subjects per every 10,000 (0.04%) males in the general population.

[68] One survey in the United Kingdom found that the majority of people with KS identify as male, however, a significant number have a different gender identity.