Some medical geneticists question whether the term "syndrome" is appropriate for this condition because its phenotype is normal and the vast majority (an estimated 97% in the UK) of 47,XYY males do not know their karyotype.
Testosterone levels (prenatally and postnatally) are normal in 47,XYY males. Most 47,XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason.
As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings. It is important to realize that this amount of variation — an average difference of 12 IQ points — occurs naturally between children in the same family. In 14 prenatally diagnosed 47,XYY boys from high socioeconomic status families, IQ scores available for 6 boys ranged from 100–147 with a mean of 120. For 11 boys with siblings, in 9 instances their siblings were stronger academically, but in one case they were performing equal to and in another case superior to their brothers and sisters.
Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47,XYY and are managed no differently than in 46,XY males. Aggression is not seen more frequently in 47,XYY males.
In some cases, the addition of an extra Y chromosome results from nondisjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics.
47,XYY was the last of the common sex chromosome aneuploidies to be discovered, two years after the discoveries of 47,XXY, 45,X, and 47,XXX in 1959. Even the much less common 48,XXYY had been discovered in 1960, a year before 47,XYY. Screening for these X chromosome aneuploidies was possible by noting the presence or absence of "female" sex chromatin bodies (Barr bodies) in the nuclei of interphase cells in buccal smears, a technique developed a decade before the first reported sex chromosome aneuploidy.
An analogous technique to screen for Y chromosome aneuploidies by noting supernumerary "male" sex chromatin bodies was not developed until 1970, a decade after the first reported sex chromosome aneuploidy. In December 1969, Lore Zech at the Karolinska Institute in Stockholm first reported intense fluorescence of the AT-rich distal half of the long arm of the Y chromosome in the nuclei of metaphase cells treated with quinacrine mustard. Four months later, in April 1970, Peter L. Pearson and Martin Bobrow at the MRC Population Genetics Unit in Oxford and Canino G. Vosa at the University of Oxford reported fluorescent "male" sex chromatin bodies in the nuclei of interphase cells in buccal smears treated with quinacrine dihdyrochloride.
Less sympathetically, Kenneth Royce's series of novels about The XYY Man partially played up to the stereotype, with an anti-hero figure William 'Spider' Scott, whose extra Y chromosome is seen in part to be responsible for his career as a highly skilled (though non-violent) cat-burglar. Royce's books were turned into a TV series in the UK which ran 3 episodes in the summer of 1976 and 10 episodes in the summer of 1977.
The short story "The Procrustean Petard," a Star Trek-based short story by Sondra Marshak and Myrna Culbreath, depicted a story where the crew of the Enterprise had their genders reversed, all but Spock who'd instead been given an extra Y chromosome. McCoy was concerned whether Spock's emotional control could handle the "hyper masculinity" the extra Y would cause, worried that he might become prone to emotional outbursts, even violence.
In the episode "Born Bad" of the show Law & Order the defense strategy for Chris Paulit, a young boy accused of beating to death another boy, is his extra Y chromosome. Experts are called to testify that the XYY syndrome is more common among the prison population.
In Larry Niven's short story "Brenda" (written in frequent collaborator Jerry Pournelle's CoDominium future history), Niven claims that the gene-engineered Sauron supersoldiers had, among many other genetic modifications, the XYY condition. In the story, the Sauron supermen had many features common to the popular XYY stereotype.