A family study of psychiatric comorbidity in girls and boys with attention-deficit/hyperactivity disorder
Introduction
Because of gender differences in attention-deficit/hyperactivity disorder (ADHD) and the limited amount of research on girls (Gaub and Carlson 1997b), the National Institute of Mental Health’s Conference on Sex Differences in ADHD concluded that inferences drawn from studies of ADHD boys could not be routinely applied to ADHD girls (Arnold 1996). Perhaps the most replicable gender difference in ADHD is its greater prevalence among boys. In clinics, the male-female ratio is about 9 to 1; in the community it is 4 to 1. The difference between the clinic and community-based gender ratios suggests that girls with ADHD are less likely to be referred for treatment. Thus, they may be under-identified and under-treated Arnold 1996, Gaub and Carlson 1997b.
A meta-analysis by Gaub and Carlson (1997b) suggested that, compared with boys, girls with ADHD tend to have greater intellectual impairments, lower levels of hyperactivity, lower rates of conduct disorder, and higher rates of mood and anxiety disorders. Comorbidity with conduct disorder is commonly associated with the high levels of family disruption and behavioral disturbance that lead to clinical referral. So, if these problems are less prevalent among ADHD girls, that could explain why girls are less likely to come to the attention of health care providers Safer and Krager 1988, Wilens and Biederman 1992.
Several studies have looked at the relationship between gender and the DSM-IV subtypes of ADHD. The DSM-IV field trials found inattentive patients were more likely to be female (Lahey et al 1994). Similar results were reported by Wolraich et al (1996), but not by several other studies Baumgaertel et al 1995, Eiraldi et al 1997, Faraone et al 1998, Gaub and Carlson 1997a, Morgan et al 1996, Paternite et al 1995.
In a large case-control study, we previously showed that, compared with non-ADHD girls, ADHD girls were more likely to have conduct, mood, and anxiety disorders, lower intelligence quotient (IQ) and achievement scores, and more impairment on measures of social, school, and family functioning (Biederman et al 1999). Although this pattern of comorbidity and dysfunction was similar to what we had seen in ADHD boys, the prevalence of both conduct disorder and oppositional defiant disorder found in ADHD girls was half of what we had seen in ADHD boys (Biederman et al 1991).
With the exception of substance use disorders, however, we also showed that the gender differences in psychopathology between ADHD boys and girls paralleled those seen for non-ADHD boys and girls. That finding suggests that the presence of ADHD does not influence gender differences in the expression of comorbid psychopathology. For substance use disorders, we found that ADHD was a significantly weaker risk factor for substance use disorders in boys than it was in girls.
This report analyzes data from two family studies of ADHD: a study of ADHD and non-ADHD boy probands (Biederman et al 1991) and a study of ADHD and non-ADHD girl probands (Biederman et al 1999). A prior report from this sample demonstrated that ADHD was equally familial in boys and girls (Faraone et al 2000). The main goal of this report was to determine if the familial aggregation of comorbid disorders due to ADHD is influenced by gender. Given that prior work found ADHD to be an equally potent risk factor for comorbidity in boys and girls, we hypothesized that the risk for comorbid disorders imparted by ADHD would not differ for families selected through boy and girl probands.
Section snippets
Subjects
We studied two groups of boys: 140 ADHD probands and 120 non-ADHD comparisons. These groups had 454 and 368 first-degree biological relatives, respectively, who provided data. We also studied two groups of girls: 140 ADHD probands and 122 non-ADHD comparisons. These groups had 417 and 369 first-degree biological relatives, respectively, who provided data. Table 1 describes the distribution of mothers, fathers, brothers, and sisters in each group.
All probands were between the ages of 6 and 18
Results
Table 2 shows demographic features of the probands and relatives. For these features we found no statistically significant gender-by-diagnosis interactions, main effects of gender or main effects of ADHD.
Table 3 presents the lifetime prevalence of psychiatric disorders among parents and siblings of the four proband groups: ADHD boys, ADHD girls, control boys, and control girls. The statistics in the column labeled “Gender × ADHD interaction” test if the familial risk for psychiatric disorders
Discussion
We have shown that the pattern of transmission of ADHD and comorbid disorders is not influenced by the proband’s gender. This is true for both the type of disorder transmitted and the degree of risk to relatives. We also found some main effects of gender. These showed that, regardless of diagnosis (ADHD vs. control subject) relatives of girl probands were at higher risk for agoraphobia, alcohol abuse, and substance abuse and at lower risk for overanxious disorder.
The finding of no interactions
Acknowledgements
This work was supported by grant RO1 MH41314-07 from the National Institute of Mental Health, Bethesda, MD to Dr. Biederman.
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