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Introduction
Initially, research in behavioural and psychiatric genetics focused on disentangling the genetic and environmental influences on traits or disorders. The findings of these studies were highly relevant in showing the influence of genetic factors in the aetiology of almost all traits and disorders. Most of these studies assumed that the effects of genes and environment act independently, meaning that the effect of an environmental risk factor does not depend on the genotype. In a seminal paper, Kendler & Eaves (1986) presented two alternative models that represent how genes and environment jointly influence variation in a trait or disorder: genotype-environment correlation (rGE) and genotype-environment interaction (GxE). rGE occurs when genes that influence a trait also influence the exposure to an environmental risk factor (Plomin et al. 1977; Kendler & Eaves, 1986). GxE occurs when the effect of exposure to environmental factors depends on a person's genotype. In the presence of GxE, individuals with a 'sensitive' genotype will be at greater risk if the predisposing environment is present than individuals with an 'insensitive' genotype (Boomsma & Martin, 2002; Rutter, 2007).
Borderline personality disorder (BPD) is characterized by emotional lability, impulsivity, interpersonal difficulties, identity disturbance, and stress-related cognitive distortion (APA, 2000). A combination of factors from various domains influences the risk to develop BPD. Many studies in clinical samples have demonstrated that traumatic life events such as sexual or physical abuse and parental divorce, loss or illness are generally more common in patients with BPD than in non-patients or patients with other personality disorders (Paris, 1997; Zanarini et al. 1997; Helgeland & Torgersen, 2004; Bandelow et al. 2005). The total number of negative life events experienced is also higher for BPD patients than for control subjects (Jovev & Jackson, 2006; Horesh et al. 2008).
Recently, several twin and twin family studies provided evidence that genetic factors explain familial clustering of BPD, with heritability estimates ranging from 35% to 45% (Distel et al. 2008 a; Kendler et al. 2008; Torgersen et al. 2008). Although many researchers and psychiatrists acknowledge the importance of both traumatic life events and biological vulnerabilities (Livesley, 2008; Distel et al. 2009 a), the joint influence of life events and genetic vulnerability on...