Papua New Guinea election violence: what has caused it and what can be done?

The massacre of 18 people in the Highlands confirmed fears that an outdated electoral roll and lack of security could lead to trouble

Voting in Papua New Guinea's elections has been overshadowed by outbreaks of violence including the massacre of 18 people in the Highlands and a group of men attacking people with machetes outside a counting centre in Port Moresby.

Voting began at the beginning of July and continues until the end of the month. But what has caused the violence and what can be done to stop it?