Some males with XYY syndrome show very few symptoms, whilst others may be more severely affected. Some men with XYY have mild symptoms and therefore are not diagnosed. XYY boys grow taller than average, they have a 'growth spurt' during childhood which results in an average height of 6 foot, 2 inches. In early childhood, XYY boys are very active, with good eating and sleeping patterns. During adolescence, they may experience severe acne.
In some cases, XYY males show learning difficulties, with slightly lower intelligence than XY males. They may have delayed speech development and have difficulties in communication.
It has been suggested that some XYY males display difficult and defiant behaviour, which usually starts in childhood. Temper tantrums are common and XYY males may be at higher risk of having problems at school. Sexual development is normal and fertility is not affected.