How can psychological adaptations be heritable?

By Fisher's fundamental theorem, selection depletes additive genetic variation. However, moderate heritabilities are invariably obtained for psychological traits, even those that have been under intense selection. Examples include sociosexuality (interest in emotionally uncommitted sex), schizophrenia and sexual orientation, which have all been subject to strong sexual selection. A number of factors can help maintain (or at least slow depletion of) genetic variation. These include antagonistic pleiotropy; geographic or temporal variability in optimal phenotypes (and hence genotypes); mutational pressure (especially in the context of parasite resistance dynamics); and existence of heritable strategic variation or morphs. I discuss the likelihood that these factors maintain heritable variation for intelligence. I then review some evolutionary hypotheses regarding variation in some specific psychological traits.