Family Studies | Twin Studies | Adoption Studies | High-Risk Studies | Mode of Inheritance | Linkage, Association, and Gene Expression Analyses
Beginning with the pioneering work of Rudin (1916), Kallmann (1946/1994), and others in the Berlin school, more than
5,000 affected individuals and relatives have been examined for
the risk of schizophrenia (Riley and Kendler 2006).
These studies, more than 25 to date, have consistently reported
elevated morbid risks for schizophrenia in the first-degree relatives
of schizophrenic probands (parents, mean = 5.6%;
sibs, 10.1%; children, 12.8%) compared with those
in the general population (0.9%), suggesting that schizophrenia
is familial (Gottesman and Shields 1982). Subsequent studies,
meeting modern criteria for the collection of family data, have
corroborated the earlier findings. The observed lower risk to parents,
at odds with Mendelian inheritance—all first-degree relatives
sharing 50% of their genes should show equal risk—may
be reflecting reproductive fitness effects.