The unilateral appropriation of Aboriginal land led to much frontier violence. Credit:Michele Mossop Australia's most distinguished economic historians were Noel Butlin, of the Australian National University, and his older brother, Syd, of Sydney University (after whom its Butlin Avenue is named). Noel backcast Australia's GDP to 1861, then began researching what the Australian economy must have been like before white settlement. He wrote up his findings in Economics and the Dreamtime: A Hypothetical History (which I wrote up in a column on April 5, 1995). As part of this research Butlin devoted much effort to estimating the size of the Aboriginal population before 1788. The anthropologist Alfred Radcliffe-Brown wrote in the Commonwealth Yearbook of 1930 that it would have been more than 250,000, maybe even more than 300,000. But Butlin's piecing together of the evidence told him this was way too low. He wrote in 1983 that it would have been 1 million or 1.5 million.

Then in 1988 some of Australia's leading archaeologists, led by John Mulvaney, argued that a more accurate estimate would be between 750,000 and 800,000. This has become accepted as "the Mulvaney consensus". Now enter Dr Boyd Hunter, of the Centre for Aboriginal Economic Policy Research at ANU. With Professor John Carmody, a physiologist at Sydney University, he published this year in the Australian Economic History Review a long paper reviewing Butlin's population estimates. The point, of course, is that the Aboriginal population declined dramatically in the early days of white settlement. We can be reasonably confident that, by 1850, the Indigenous population was only about 200,000. Thus backcasting the figures to 1788 involves determining the main factors that led to the loss of Aboriginal lives and estimating how many lives they took, then adding them back. So the paper is a kind of whodunit. One factor springing to the modern mind is that the unilateral appropriation of Aboriginal land led to much frontier violence, which started shortly after the arrival of the First Fleet and persisted well into the 20th century.

"Like any war, declared or otherwise, the conflict led to many deaths on both sides," the authors say. But even the controversial historian, Henry Reynolds, estimated the number of violent Aboriginal deaths at as many as 20,000, making this only a small part of the explanation. Butlin allows for Aboriginal "resource loss", where tribes' loss of productive members and land used for sustenance led to people dying of "starvation or dietary-related diseases". Butlin's calculation implies this factor would have involved as many as 120,000 people. That's still not the biggest part of the story. No, the big factor is the spread of introduced diseases. Such as? Tuberculosis, bronchitis and pneumonia, not to mention venereal disease. But the big one is smallpox. Butlin and others have assumed that it spread rapidly around Australia along the extensive pre-existing Aboriginal trading routes after its first recorded outbreak in Port Jackson in April 1789. In 2002, however, the former ANU historian Judy Campbell argued in her book, Invisible Invaders, that it was brought to Northern Australia by the Macassan coastal traders following its outbreak in Sumatra in 1780, then spread across the continent, reaching Port Jackson by early 1789.

This is where Hunter – no doubt relying heavily on the expertise of Carmody – brings to bear modern medical understanding of the infectiousness and mortality rates of various diseases. Although smallpox has a high rate of mortality – between 30 and 60 per cent of those who contract it – it's not highly infectious. This means it happens most in densely populated areas and doesn't spread rapidly to distant areas. This casts doubt on Campbell's theory that smallpox spread rapidly from lightly populated Northern Australia to densely populated NSW. But it also casts doubt on Butlin's theory that smallpox spread rapidly from Sydney to the rest of Australia via Aboriginal trading routes. So what's Hunter and Carmody's theory? Are you sitting down? Gathering all the suspects in a room, detective Hunter deftly turns the finger of guilt from smallpox to the so-far unsuspected chickenpox. The two are quite separate diseases, but this wasn't well-known in the 1780s. And since they both give rise to rashes or spots around parts of the body, many people may not have been able to tell the difference.