Reduced fertility in patients' families is consistent with the sexual selection model of schizophrenia and schizotypy.
BACKGROUND
Schizophrenia is a mental disorder marked by an evolutionarily puzzling combination of high heritability, reduced reproductive success, and a remarkably stable prevalence. Recently, it has been proposed that sexual selection may be crucially involved in the evolution of schizophrenia. In the sexual selection model (SSM) of schizophrenia and schizotypy, schizophrenia represents the negative extreme of a sexually selected indicator of genetic fitness and condition. Schizotypal personality traits are hypothesized to increase the sensitivity of the fitness indicator, thus conferring mating advantages on high-fitness individuals but increasing the risk of schizophrenia in low-fitness individuals; the advantages of successful schzotypy would be mediated by enhanced courtship-related traits such as verbal creativity. Thus, schizotypy-increasing alleles would be maintained by sexual selection, and could be selectively neutral or even beneficial, at least in some populations. However, most empirical studies find that the reduction in fertility experienced by schizophrenic patients is not compensated for by increased fertility in their unaffected relatives. This finding has been interpreted as indicating strong negative selection on schizotypy-increasing alleles, and providing evidence against sexual selection on schizotypy.


METHODS
A simple mathematical model is presented, showing that reduced fertility in the families of schizophrenic patients can coexist with selective neutrality of schizotypy-increasing alleles, or even with positive selection on schizotypy in the general population. If the SSM is correct, studies of patients' families can be expected to underestimate the true fertility associated with schizotypy.


CONCLUSIONS
This paper formally demonstrates that reduced fertility in the families of schizophrenic patients does not constitute evidence against sexual selection on schizotypy-increasing alleles. Futhermore, it suggests that the fertility estimates derived from extant studies may be biased to an unknown extent. These results have important implications for the evolutionary genetics of psychosis.
Department of Psychology, Center for Cognitive Science, University of Turin, Torino, Italy. marco.delgiudice@unito.it
Marco
M
Del Giudice
D000483:Alleles; D019143:Evolution, Molecular; D005192:Family Health; D005260:Female; D005298:Fertility / Q000235:genetics*; D017353:Gene Deletion; D020022:Genetic Predisposition to Disease; D006801:Humans; D007246:Infertility / Q000235:genetics*; D008297:Male; D008957:Models, Genetic; D015233:Models, Statistical; D008962:Models, Theoretical; D012559:Schizophrenia / Q000235:genetics*; D012569:Schizotypal Personality Disorder / Q000235:genetics*
Acta Psychiatr Scand. 1997 Oct;96(4):260-4
9350954
Schizophr Res. 2008 Jul;102(1-3):1-18
18514488
Epidemiol Rev. 2008;30:67-76
18480098
Acta Psychiatr Scand. 1993 Jun;87(6):400-4
8356891
Front Psychol. 2010 Aug 30;1:41
21833210
Addiction. 2005 May;100(5):612-8
15847618
Schizophr Bull. 2003;29(3):587-93
14609251
Schizophr Res. 1996 Sep 18;21(3):151-60
8885043
Hum Nat. 2006 Mar;17(1):50-73
26181345
Acta Psychiatr Scand. 1999 Jun;99(6):441-6
10408266
Psychol Med. 2009 Oct;39(10):1667-76
19265569
Am J Psychiatry. 2003 Mar;160(3):460-3
12611825
Schizophr Res. 1999 Dec 21;40(3):173-7
10638855
Schizophr Res. 2004 Sep 1;70(1):101-9
15246469
Schizophr Res. 2010 May;118(1-3):224-31
20153952
Acta Psychiatr Scand. 1998 Nov;98(5):348-53
9845171
Nature. 1964 Oct 17;204:220-1
14212411
Proc Biol Sci. 2006 Mar 7;273(1586):611-5
16537133
Neurosci Biobehav Rev. 2004 Mar;28(1):41-53
15036932
PLoS Med. 2005 May;2(5):e141
15916472
PLoS One. 2009 Nov 11;4(11):e7799
19911060
Schizophr Res. 2007 Mar;91(1-3):238-45
17275261
Schizophr Res. 2001 Mar 1;47(2-3):233-41
11278140