Most experts agree that there is no single cause of borderline personality disorder (BPD). It is likely to be caused by a combination of factors.

These include:

• genetics – genes you inherit from your parents may make you more vulnerable to developing BPD, given certain environmental factors (see below)
• neurotransmitters – these are "messenger chemicals" used by your brain to transmit signals between brain cells; certain neurotransmitters can have a significant effect on mood and behaviour
• neurobiology – this term describes the structure and function of your brain and nervous system; it appears that some people with BPD have a number of regions in the brain with abnormal structure and function
• environmental factors – events that happened in your past, such as your relationship with your family, appear to play an important role in BPD

These are explained in more detail below.

Genetics

The strongest evidence that genetics may play a role in BPD is research that studied twins.

One study found that if one identical twin had BPD, there was a two-in-three chance that the other identical twin would also have BPD.

However, these results have to be interpreted with caution, and there is no evidence there is a gene for BPD.

Firstly, you may be more likely to develop certain personality traits. For example, you may inherit from your parents a tendency to be aggressive and emotionally unstable, rather than BPD itself.

Secondly, most identical twins grow up in the same household and in the same family environment, so they will share many environmental factors.

Neurotransmitters

It is thought that many people with BPD may have altered functioning of a neurotransmitter called serotonin in their brain.

Altered serotonin activity in the brain has been linked to depression, aggression and difficulty in controlling destructive urges.

There is also some limited evidence that some people with BPD also have altered functioning of two other neurotransmitters, called dopamine and noradrenaline, that may be associated with emotional instability.

Neurobiology

Researchers have used magnetic resonance imaging (MRI) scans to study the brains of people with BPD. MRI scans use strong magnetic fields and radio waves to produce a detailed image of the inside of the body.

The scans revealed that in many people with BPD, three parts of the brain were either smaller than expected or had unusual levels of activity. These parts were:

• the amygdala – which plays an important role in regulating emotions, especially the more "negative" emotions, such as fear, aggression and anxiety
• the hippocampus – which helps regulate behaviour and self-control
• the orbitofrontal cortex – which is involved in planning and decision making

Problems with these parts of the brain may well contribute to symptoms of BPD.

The development of these parts of the brain is affected by your early upbringing (see below). These parts of your brain are also responsible for mood regulation, which may account for some of the problems people with BPD have in close relationships.

Environmental factors

A number of environmental factors seem to be common and widespread among people with BPD. These include:

• being a victim of emotional, physical or sexual abuse
• being exposed to chronic fear or distress as a child
• being neglected by one or both parents
• growing up with another family member who had a serious mental health condition, such as bipolar disorder or a drink or drug misuse problem

A person's relationship with their parents and family has a strong influence on how they come to see the world and what they believe about other people.

Unresolved fear, anger and distress from childhood can lead to a variety of distorted adult thinking patterns, such as idealising others, expecting others to be a parent to you, expecting other people to bully you, and behaving as if other people are adults and you are not.