The Minnesota Multiphasic Personality Inventory-2 (MMPI-2) 1
represents one of the most commonly used measures of psychopathology and personality. Despite this, only a limited number of studies have examined the heritability of the MMPI-2 scales 12;15-17
. Such studies are needed given that understanding patterns of co-occurring psychiatric symptomatology may aid in the creation of refined phenotypes for psychiatric disorders. For example, MMPI-2 scales have been shown to assess a portion of the genetic susceptibility to schizophrenia 87
. Results from the present study suggest that specific MMPI-2 scales can be used to identify genetically informative clinical and personality traits that may represent useful phenotypes for molecular genetic studies of alcohol dependence.
Primary analyses demonstrated that each of the MMPI-2 scales examined showed significant evidence of heritability. When the alcohol dependence diagnosis was not included as a covariate, the largest estimates were observed for the DEP and Hy, Hs, Sc, and Pt. In addition, the D, Pd, Pa, Ma, Si, ASP and MAC-R scales all exhibited heritability estimates above 0.25 suggesting substantial genetic influences underlie these traits. When the alcohol dependence diagnosis was included as a covariate in the model, several of the scales (Hs, D, Hy, Pd, Pa, Pt, Sc, DEP) showed decreases in their heritability estimates, though the estimates remained significant for each scale. In contrast, the heritability of the alcohol dependence diagnosis showed much smaller changes, in general, when estimated using each of the MMPI-2 scales as a covariate in turn.
The stability of the heritability of alcohol dependence when controlling for MMPI-2 scores relative to the larger declines in MMPI-2 scale heritabilities when alcohol dependence is controlled for suggests a portion of the variance in MMPI-2 scores attributed to genetic influences can in fact be explained by the presence or absence of alcohol dependence. Nonetheless, three scales, ASP, DEP and MAC-R, led to moderate increases (30–35%) in the heritability of the alcohol dependence diagnosis when used as covariates. This suggests that the ASP, DEP, and MAC-R scales may explain some of the non-genetic variance in the alcohol dependence diagnosis when utilized as covariates, and thus serve to produce a more homogeneous and more heritiable alcohol dependence phenotype. Given that the ASP and DEP scales measure antisocial personality traits and depressive symptoms, respectively, this result is not unexpected due to the previous research on alcoholic subtypes 20;88
. Thus, these results support the use of specific MMPI-2 scales as covariates and the creation of alcohol dependence subtypes based on these scales for use in molecular genetic studies of alcohol dependence.
The findings that the MMPI-2 scales continue to show significant evidence of underlying genetic influences after controlling for the presence of alcohol dependence also add to the previous literature suggesting that the MMPI-2 yields heritable estimates of personality and psychopathology 12;15-17
. For example, Viken and Rose 17
reported significant genetic influences underlie each of the MMPI-2 basic scales with estimates of additive genetic influences that ranged from 0.31–0.57. They also reported an absence of shared genetic influences on the MMPI scales scores, which has important implications for the findings reported herein. Heritability estimates in the current study were generated using sibling pair correlations as the primary unit of measurement. Because monozygotic twin pairs were not included, the effects of genetic influences could not be distinguished from the shared environment of the sibling pairs, which could positively bias the heritability estimates. Thus, the absence of shared environmental influences on the MMPI scales suggested by Viken and Rose 17
indicate that the heritability estimates reported in the present study were unlikely to be biased in this manner, and therefore, represent valid measurements of the relative contributions of genetic influences on the MMPI-2 scales.
Secondary analyses were conducted in order to further characterize MMPI-2 scale scores in the UCSF Family sample. These analyses showed that UCSF Family study participants with an alcohol dependence diagnosis scored higher than the sample used to provide the norms for the MMPI-2 1
on each of the scales examined, but it is noteworthy that the psychopathological symptoms reported by most participants in the UCSF sample were in the subclinical range. Thus, these participants reported only mild impairment relative to the normative sample. Further analyses suggested participants in the UCSF sample diagnosed with alcohol dependence exhibited lower scores on the D, Pt, and Ma basic scales and the ANX, DEP, and MAC-R supplementary scales relative to published data obtained from participants undergoing inpatient treatment for a substance use disorder. These results were expected given that the present study recruited participants from outpatient treatment programs and required only a positive history for alcohol dependence. Additionally, these results suggest that participants diagnosed with alcohol dependence in the present sample exhibited MMPI-2 scale scores that were intermediate to the MMPI-2 normative population and published data from inpatient participants diagnosed with a substance dependence disorder.
Further analyses demonstrated that among UCSF participants without an alcohol dependence diagnosis, the prevalence rates of individuals experiencing significant symptoms of psychopathology associated with certain disorders as indexed by cut off scores on the MMPI-2 were similar to prevalence rates for those disorders that have been reported in surveys of the general population. For example, 11% of participants reported experiencing significant symptoms of anxiety in the current sample, which is comparable to the population prevalence of anxiety disorders in the general population of 11% 62
. Similar results were found for other disorders examined, depression - 10% in the UCSF sample vs. 9% in epidemiological samples 62
, antisocial personality disorder - 5% vs. 4% 59
, respectively, and schizophrenia spectrum disorders - 3% vs. 4–5%, respectively 85;86
The findings reported in the present study have important implications for molecular genetic studies of alcohol dependence, but there are limitations that should be noted. First, the UCSF sample contains an over-representation of female participants given prevalence estimates suggesting males are approximately twice as likely as females to develop alcohol dependence. Attempts were made to correct for this by including gender as a covariate in all analyses when appropriate, but further studies will be needed to determine whether the reported findings will generalize to other populations. Second, it should be noted that the etiology of alcohol dependence and the development of personality traits and co-occurring psychopathology represent multi-factorial processes likely involving a range of biological as well as environmental risk factors. The influence of such environmental variables (e.g., exposure to trauma, aspects social environment) and their relation to the findings presented herein were not examined, but represent interesting lines of inquiry for future research. Third, demographic variables such as age were not available at the participant level for the MMPI-2 normative sample1
and the alcohol dependence inpatient sample described by McKenna and Butcher83
, and thus, could not be controlled for when comparing MMPI-2 scores. Nonetheless, comparisons of alcohol dependent and non-alcohol dependent subjects in the UCSF sample remained significant whether age was included as a covariate or not and the corresponding effect sizes showed changes that did not exceed ΔR2
=.01 with the exception of the ANX scale (ΔR2
=.04). Fourth, the present study used cutoff scores applied to specific MMPI-2 scales as proxies for DSM-IV diagnoses which were used to compare rates of psychopathology in the UCSF sample to prevalence rates in the general population. This assessment procedure could potentially produce less stringent diagnoses than might be obtained using structured interviews. Nonetheless, there is a substantial literature supporting the use of the MMPI-2 and demonstrating its reliability and validity as a measure of psychopathology and personality 89;90