Background Familiar liability in the functional psychoses had traditio
nally been examined by comparing mutually exclusive diagnostic categor
ies. This study examines overlapping psychopathological dimensions in
relation to familial morbid risk of psychosis. Method We tested for as
sociations between seven factor-analysis derived psychopathological di
mensions and familial morbid risk of psychosis, in a sample of 150 pat
ients with recent-onset function al psychosis and 548 of their first-d
egree relatives. Results A syndrome characterised by affective bluntin
g and insidious and early onset of illness, non-specifically predicted
psychosis in the first-degree relatives, whereas a manic syndrome spe
cifically predicted affective psychosis in the relatives. No other mai
n effects were observed, but there were interactions with proband diag
nosis: a syndrome characterised by bizarre behaviour, inappropriate af
fect, catatonia and poor rapport predicted psychosis in relatives of s
chizophrenic probands, and a syndrome of depressive symptoms predicted
psychosis in relatives of schizoaffective probands. Positive symptoms
were not associated with illness in the relatives. Conclusions Geneti
c effects in the functional psychoses may comprise nonspecific compone
nts that canalise a general, early-onset, affective blunting phenotype
and several other, more specific, influences on phenotypic variation.