Using data from the borderline subscale of the Personality Assessment Inventory Borderline Features Scale (PAI-BOR) collected in two Dutch cohorts (N=7125), the Netherlands Twin Register and The Netherlands Study of Depression and Anxiety, we show that heritability of the PAI-BOR total score using genome-wide single-nucleotide polymorphism (SNPs) is estimated at 23%, and that the genetic variance is substantially higher in affect instability items compared with the other three subscales of the PAI-BOR (42.7% vs non-significant estimates for self-harm, negative relations and identity problems).