Work by Neville Green in his book "Broken Spears" has shown how Aboriginal culture could not explain the high death rates associated with European infections, and believed that Aboriginal sorcery was involved, leading to rising numbers of reprisal spearing and killings within the Aboriginal community. Coupled with the declining birth rates, these factors led to a collapsing population in those areas nearby European settlement. In addition to white killings and massacres in Fremantle and elsewhere, the arrival of Europeans saw many deaths from diseases to which Aboriginal people had no resistance. These were interpreted as sorcery within traditional culture and led to "pay-back" vendettas, which increased mortality of those in closest contact with Europeans.