Objective: To determine the risk of the lesser variant (or PDD-like traits) in the biological
and nonbiological second- and third-degree relatives of PDD probands using a screening
questionnaire and to investigate the extent to which the risk of the lesser variant differs
according to various characteristics of the proband. Method: The sample consists of a series
of 34 nuclear families with 2 affected PDD children (multiplex, MPX), 44 families with a
single PDD child (simplex, SPX), and 14 families who adopted a PDD child. Data on
characteristics of the lesser variant in 1362 biological and 337 nonbiological second- and
third-degree relatives were collected from parents by telephone interview and from several
maternal and paternal relatives by questionnaire. Results: All components of the lesser
variant were more common in biological relatives (BR) than nonbiological relatives (NBR),
confirming the familial aggregation of the traits. Proband characteristics associated with an
increased risk of the lesser variant in relatives were a higher level of functioning and coming
from a MPX family. Conclusions: These findings on the familial aggregation of the lesser
variant suggest that the genes for PDD also confer susceptibility to the lesser variant and that
PDD may be a genetically heterogeneous disorder.