The incidence of ADHD is twice as great in boys relative to girls (Kaufmann, 2009). There are also various differences in male and female presentations of the disorder. Boys tend to develop the Combined Type of the disorder and tend to show greater disruptive behaviors, while girls are more likely to develop the Inattentive Type (Biederman et al., 2002). Sex-based discrepancies in cognition in ADHD have been somewhat inconclusive, although boys have been found to have greater deficits in inhibition (Gunther, Herpertz-Dahlmann, & Konrad, 2010) and processing speed (Rucklidge & Tannock, 2001). The etiology of the difference in ADHD incidence is unknown as the etiology of the disorder itself is unknown. While ADHD is theoretically associated with neuroanatomical dysfunction, few studies specifically examine sexual dimorphisms in neuroanatomy. One report found smaller prefrontal and premotor brain volumes in boys with ADHD (Mostofsky, Cooper, Kates, Denckla, & Kaufmann, 2002). Regardless of the true underlying etiology, boys are disproportionately referred for clinical services (Willcutt & Pennington, 2000) and are more likely to use stimulants even after symptom characteristics and severity are controlled (Miller, Kohen, & Johnston, 2008). Thus, whether boys are simply over-diagnosed due to greater disruptive behavior or whether there is a true sex-linked difference is still a matter of much debate.

Reading disorders (or disabilities), commonly labeled dyslexia, are more prevalent in boys than girls (D’Amato, Dean, & Rhodes, 1998). However, studies that incorporate only school-based referral rates may be biased (American Psychiatric Association, 2000). Neuropsychological data has been informative in this area. Boys may have relatively greater difficulty on language tasks relative to girls, and phonemic awareness, thought to be a core cognitive skill needed for successful reading in alphabetic languages, is one such skill in which boys lag behind girls (Meneses, Lozi, Souza, & Assencio-Ferreira, 2004). This finding has been corroborated using neuroimaging data which has demonstrated greater gyrification and relatively larger left hemisphere volumes in language-related areas (Luders et al., 2004). Interestingly, one of the methodological limitations in the study of dyslexia across the sexes is the frequent lack of case ascertainment in girls (Grabowska & Bednarek, 2004). Overall, the performance of boys in the educational system has been one of recent concerns, spurring initiatives designed to improve their performance (Rich, 2014).

The male predominance in epidemiological samples of ASD has held over time, with ratios being three to four times higher in males than in females (Chakrabarti & Fombonne, 2005). There is, however, an interaction between gender and severity as the proportion of females in samples of individuals with more severe forms of the disorder are higher than in samples of individuals with more mild forms (Volkmar, Szatmari, & Sparrow, 1993). Despite these sex-linked differences in prevalence, when level of intellectual functioning is controlled, disorder characteristics are similar across the sexes (Volkmar et al., 1993). The cause of individuals with ASD is unknown. As part of the extreme male brain theory, Baron-Cohen, Knickmeyer, and Belmonte (2005) have theorized that exaggerations of male-typical behavior manifest as an individual with ASD. Baron-Cohen, Richler, Bisarya, Gurunathan, and Wheelwright (2003) have suggested that this occurs due to increased levels of prenatal androgens which produce excessive masculinization of the brain. A corollary of the extreme male behavior theory is that the neuroanatomy of individuals with ASD represents extremes of normal male neuroanatomy (Baron-Cohen et al., 2005). While the larger brain size observed in males is perhaps one of the most common neuroanatomical findings in ASD, there is little available data on regional differences and further, direct evidence for the association between testosterone and ASD diagnosis is lacking. Genetics have also been studied in individuals with ASD and may contribute to the differences in incidence rates of individuals with ASD. While apparent male-to-male transmission of individuals with ASDs rules out X chromosome linkage as the dominant mode of transmission, studies in this area continue due to the connections between neurogenetic disorders like Fragile X and ASD.

Differences across the sexes are consistent both in the prevalence and patterns of aggressive and antisocial behavior. Boys are more likely than girls to be diagnosed with Oppositional Defiant Disorder or Conduct Disorder and men are far more likely to be diagnosed with Antisocial Personality Disorder, and this discrepancy is so great that it has raised concerns about the validity of the diagnostic criteria in girls and women (American Psychiatric Association, 2013). In general, boys tend to externalize their anger as physical aggression and other forms of antisocial behavior (Su, Simons, & Simons, 2011), and male adolescents are significantly more accepting of violence and aggressive methods of conflict resolution than are female adolescents (Garaigordobil, Maganto, Perez, & Sansinenea, 2009). The etiology of the sex-linked differences in aggression and antisocial behavior appears multifactorial. Not only do males and females have different normative levels of sex hormones, but their reactions to increases in such hormones also differ. Testosterone is associated with increased aggression and provocation in boys, particularly at puberty, whereas in girls it better predicts affectivity (Azurmendi et al., 2006). Affective empathy, which in non-pathological samples is often lower in boys, is a risk factor for antisocial behavior in males but not in females when it is deficient (Dadds et al., 2009). Sociocultural influences are also important. Boys are more likely to be competitive in group activities while girls are concerned about maintaining social harmony (Maccoby, 2002). In adulthood, adherence to traditional masculine values is associated with violence-related attitudes and behaviors (Jakupcak, Lisak, & Roemer, 2002). All of these findings have enormous societal impact, as demonstrated by the fact that in the United States, most murders and the overwhelming majority of sexual crimes are committed by men (Spratt, 2000).

In the United States, boys are overrepresented in special education classes. In a national survey, Coutinho and Oswald (2005) found that boys were 1.33 times more likely to be identified as being Mentally Retarded, 2.04 times more likely to be identified as having a Learning Disability, and 3.43 times more likely to be identified as having a Serious Emotional Disturbance. However, at the state level, there were substantial variations in the rates of gender-discrepancies in learning disability and serious emotional disturbance classifications. These variations would appear to suggest that at least some of the higher rates in boys are due to biases in referral and identification rates (MacMillan, Gresham, Lopez, & Bocian, 1996). A rejection of school attachment and a negative attitude toward schooling, whether or not accompanied by a developmental deficit, might conceivably have resulted in many more boys being labeled within the special education system. However, recent data has shown that a rejection of school values is not limited to the stereotype of the troublemaking male, but in fact extends in various forms to other school demographics across genders (Lyng, 2009). At present, the cause of the overrepresentation of boys in the special education system is not fully understood and may only be partially attributable to true underlying differences in psychopathology prevalence.

While cross-national studies do not suggest that men are any happier than women (Simon, 2008), they seem less susceptible to depression. Furthermore, in samples of men who report depression, they appear to experience a milder version of the disorder, as reflected by a later age at onset (Marcus et al., 2005), and less symptom persistence (Barry, Allore, Guo, Bruce, & Gill, 2008), cognitive impairment (Sarosi, 2011), negative perceptual bias (Bos et al., 2005), and comorbid anger and anxiety (Scheibe, Preuschhof, Cristi, & Bagby, 2003). The overall incidence of bipolar disorder is similar across the sexes (Lloyd et al., 2005). However, men with bipolar disorder also suffer from less recurrence (Suominen et al., 2009), and are less likely to develop rapid cycling (Berk & Dodd, 2004), and severe psychotic symptoms (Braunig, Sarkar, Effenberger, Schoofs, & Kruger, 2009). Various sociocultural explanations for the differences in unipolar depression have been proposed which tend to focus on the greater societal adversity faced by women and their reaction to such adversity (Nolen-Hoeksema & Hilt, 2008). Recent neuroimaging data have shed light on the neuroanatomical basis of this sex-linked difference in response to adversity (Yuan et al., 2009). The lack of a social bias in men may also serve a protective function. In comparison with women, men are less likely to come into contact with others with personal problems (Schuster, Kessler, & Aseltine, 1990), and perhaps as a result are less likely to internalize the problems of others. Men’s gender roles also appear to contribute to differential susceptibility to depression and bipolar disorder and the behaviors associated with such disorders. Thus, men with bipolar disorder exhibit lower levels of self-perceived masculinity, while those with higher masculinity are less likely to start or continue with treatment (Sajatovic, Micula-Gondek, Tatsuoka, & Bialko, 2011). In many societies men are overrepresented in cases of suicide (Moscicki, 1997) despite having lower depression prevalence, suggesting that gender roles may influence how stress is experienced and processed. While the evidence for an association between sex hormones and depression is likely stronger in women than men, reduced testosterone in men has been associated with depressive disorders (Seidman & Walsh, 1999). Moreover, the association between depression and testosterone appears to partly depend on androgen receptor genotype (Seidman, 2001).

Men are also more resilient to anxiety disorders (McLean & Anderson, 2009), although this resilience varies with the anxiety disorder subtype. It appears that obsessive-compulsive and/or social anxiety disorders are those with the least amount of sex-linked discrepancies in prevalence. Sex-linked differences in anxiety disorder symptoms are subtle but consistent as diagnosed men are rated as less anxious and depressed (Oei, Wanstall, & Evans, 1990), and are more concerned about the social (and not physical) consequences of anxiety; in women the opposite pattern is true (Foot & Koszycki, 2004). Protective factors exist in men, as they report lower levels of worry and rumination (Robichaud, Dugas, & Conway, 2003). Sex-linked physiological reactions to stress vary considerably with the stressor. For instance, men respond to adrenergic agents with more intense vasoconstriction than women (Ludwig, Vernikos, Wade, & Convertino, 2001) but may also show less cardiovascular reactivity to stress (Stoney, Davis, & Matthews, 1987). As is the case with individuals with a bipolar disorder, higher levels of masculinity are also associated with lower levels of social anxiety (Moscovitch, Hofmann, & Litz, 2005).

The incidence of schizophrenia is comparable in men and women (Seeman, 2009). Concerning symptom expression, men exhibit more deficits in social withdrawal (Mulligan & Lavender, 2010). Men also have greater deficits in social cognition (Bozikas, Kosmidis, Kiosseoglou, & Karavatos, 2006; Van’t Wout et al., 2007), and a greater degree of premorbid social isolation (Haas & Sweeney, 1992). These gender differences in symptom presentation have particular significance because of their association with functional status. That is, negative symptoms such as social withdrawal and blunted affect, which are more pronounced in men, are the most predictive of poorer outcome, and in fact, men tend to have worse functional statuses. Various theories have been proposed to account for schizophrenia etiology (Taylor & Langdon, 2006), and given the male dominance of specific developmental disorders, theories of schizophrenia that emphasize neurodevelopmental and neuroanatomical anomaly may implicate sex-linked differences.

Sex differences in the prevalence and pattern of substance abuse vary with age and with the specific substance of abuse. In adulthood, men are at their highest risk for alcohol and drug abuse/dependence; over the course of a lifetime, nearly a third of all men will develop an abuse or dependence disorder (Kessler & Walters, 2002), rates which far exceed all other mental disorders in men. Some reports suggest that while women may use more alcohol than men, men are more likely to abuse alcohol (Cotto et al., 2010). Definitive data on sex differences in the brain mechanisms behind use, abuse, and dependence are not available. Greater central dopaminergic reward system activity has been implicated in male amphetamine-stimulated dopamine release (Pogun & Yararbas, 2009), although further research is needed with other drugs of potential abuse. Attempts to correlate neuroimaging findings like white matter volume, with risk for substance abuse across the sexes have been unsuccessful (Silveri, Tzilos, & Yurgelun-Todd, 2008). Sexually dimorphic patterns of cognitive deficits in substance abuse and dependence vary by substance and dosage. For instance, men display greater impairment of inhibition when receiving an acute dose of alcohol relative to women (Fillmore & Weafer, 2004), whereas deficits are greater in women with heavy use (Nederkoorn, Baltus, Guerrieri, & Wiers, 2009). The latter findings may be attributed to the fact that equivalent brain atrophy exists across the sexes despite greater usage in men (Mann et al., 2005). Of note, the societal cost of substance use is far higher in men than in women in at least one respect: the majority of episodes of intimate violence committed by men are associated with acute alcohol or drug use (Greenfield, 1998).

Men may be more susceptible to developing epilepsy and unprovoked seizures (Kaufmann, 2009) and males tend to predominate in acute symptomatic seizure cases. While men are generally overrepresented in all epilepsy samples compared to women (Amatniek, Sorra, Frey, & Hauser, 2009), the incidence of specific epilepsy syndromes in individual reports may be higher in both women and men. The exception appears to be the epilepsy risk in the oldest age groups, which may be associated with the increased risk of stroke (Amatniek et al., 2009). There is little information available concerning male-specific patterns of cognition and behavior in epilepsy. Most descriptions of sex in epilepsy focus on the unique ways in which the assessment and treatment of women with epilepsy vary by life stage (Fletcher-Janzen, 2009). Typically, effect sizes for cognitive impairment in epilepsy are much larger for epilepsy etiology than for gender. And because men have more accidents they are more prone to cause their own onset of epilepsy, often as a secondary disorder.

In childhood and young adulthood, males are twice as likely as females to experience a traumatic brain injury (Kraus & McArthur, 1998). Sexually dimorphic outcomes of traumatic brain injury depend upon the operational definition of outcome. Sex differences in mortality rates after traumatic brain injury are inconsistent (Gujral, Stallones, Gabella, Keefe, & Chen, 2006; Harrison-Felix et al., in press; Klauber et al., 1989), although men are more likely to have difficulty integrating into the community post-injury (Reid-Arndt, Nehl, & Hinkebein, 2007). Sex differences in neuropsychological functioning following traumatic brain injury are somewhat equivocal across several cognitive domains. While some reports find that men possess better memory after injury when injury severity and premorbid level of intellectual functioning are taken into consideration (Liossi & Wood, 2009), more commonly, women do better on memory tasks post-injury (Moore, Ashman, Cantor, Krinick, & Spielman, 2010). Findings in other cognitive domains such as attention are equivocal (Moore et al., 2010; Ratcliff et al., 2007). Neurobiological mechanisms and hormonal factors may account for sex differences or the lack thereof in cognitive functioning after traumatic brain injury. That is, men appear to have more focal organization of brain function (Farace & Turkheimer, 1996) and thus may be less susceptible to the effects of diffuse brain injury. On the other hand, progesterone may aid in recovery from traumatic brain injury (Wright et al., 2007).