Borderline personality disorder (BPD) is the most common and the most serious of the personality disorders (PDs) in clinical practice. This chapter overviews sex differences in BPD, covering clinical presentation, longitudinal course, aetiological factors and neurobiological underpinnings. Cross-sectional baseline data from the Collaborative Longitudinal Study of Personality Disorders (CLPS) comparing 175 women with 65 men with BPD found men with BPD to be more likely to present with substance use disorders, along with schizotypal, narcissistic and antisocial personality disorder (ASPD). The aetiological underpinnings of sex differences in PDs in general and BPD in particular are complex, with multi-level interactions of genetic and environmental parameters acting at certain vulnerable stages of neural, emotional and social development. Neuropsychological investigation suggests a dysfunctional prefrontal circuit in impulsive aggression. Three positron emission tomography (PET) studies have found prefrontal cortical hypo metabolism in BPD compared to healthy controls.

The biological approach to functional psychosis began to gain ground and went on to become the dominant explanatory paradigm. In schizophrenia, studies of memory now rival, and may even have overtaken, the other main area of neuropsychological investigation, frontal or executive function. In affective psychosis, the long-standing observation that memory may be affected in depression has gone from being automatically considered secondary to motivational or other factors, to a phenomenon deserving recognition and investigation in its own right. Both schizophrenia and manic-depressive psychosis are associated with memory deficits as part of a wider pattern of cognitive impairments, which can occasionally become severe. The features of the impairment in both disorders, plus other circumstantial evidence, suggest that it is 'biological' in nature, but not 'neurological', that is, that it has different underlying causes than those normally encountered in neurology.