BACKGROUND: Adolescent idiopathic scoliosis occurs between two and ten times 
more frequently in females than in males. The exact cause of this sex 
discrepancy is unknown, but it may represent a difference in susceptibility to 
the deformity. If this difference is attributable to genetic factors, then males 
with adolescent idiopathic scoliosis would need to inherit a greater number of 
susceptibility genes compared with females to develop the deformity. Males would 
also be more likely to transmit the disease to their children and to have 
siblings with adolescent idiopathic scoliosis. Such a phenomenon is known as the 
Carter effect, and the presence of such an effect would support a multifactorial 
threshold model of inheritance.
METHODS: One hundred and forty multiplex families in which more than one 
individual was affected with adolescent idiopathic scoliosis were studied. These 
families contained 1616 individuals, including 474 individuals with adolescent 
idiopathic scoliosis and 1142 unaffected relatives. The rates of transmission 
from the 122 affected mothers and from the twenty-eight affected fathers were 
calculated, and the prevalence among siblings was determined in the nuclear 
families of affected individuals.
RESULTS: The prevalence of adolescent idiopathic scoliosis in these multiplex 
families was lowest in sons of affected mothers (36%, thirty-eight of 105) and 
highest in daughters of affected fathers (85%, twenty-two of twenty-six). 
Affected fathers transmitted adolescent idiopathic scoliosis to 80% 
(thirty-seven) of forty-six children, whereas affected mothers transmitted it to 
56% (133) of 239 children (p < 0.001). Siblings of affected males also had a 
significantly higher prevalence of adolescent idiopathic scoliosis (55%, 
sixty-one of 110) compared with siblings of affected females (45%, 206 of 462) 
(p = 0.04).
CONCLUSIONS: This study demonstrates the presence of the Carter effect in 
adolescent idiopathic scoliosis. This pattern can be explained by polygenic 
inheritance of adolescent idiopathic scoliosis, with a greater genetic load 
required for males to be affected.