The postal survey is finally done.

Australians may have backed same-sex marriage at a national level, but the country's 150 electorates have returned radically different levels of support.

Results show there is a huge discrepancy between the places that want same-sex marriage the most (Melbourne and Sydney), and least (Blaxland in western Sydney).

And although many parts of the country participated as predicted — in line with polls and demographic breakdowns — some surprises have also raised questions about what makes Australians believe the things they do.

Spot the same-sex marriage supporter

So how predictable were the ways Australians filled out and returned their forms?

The turnout, at close to four out of every five eligible participants, was high for an optional survey by international standards.

The response however, at 61.6 per cent in support of same-sex marriage, was in line with polling in the period of the survey.

And most electorates responded as predicted based on their demographic make-up.

Supporters of equal rights for same-sex couples are more likely to be female, young, university-educated, Australian-born, not religious and high-income earners.

The typical SSM supporter? female

in their 20s or 30s

graduate of year 12 or higher

English-speaking

not religious

on a high income

These six characteristics have been identified as significant in analysis by Dr Francisco Perales from the University of Queensland of the Household, Income and Labour Dynamics in Australia (HILDA) Survey.

ABS Census data tells us how common these characteristics are among the people living in each of Australia's 150 federal electorates.

Sydney, for example, is top among all electorates for the proportion of residents in their 20s and 30s, and residents that finished year 12.

Blaxland, at the other end of the scale, is ranked in the bottom 10 of all electorates in three different categories: the proportion of residents who have no religion, those who earn at least $1,500 per week and those who are female.

Most electorates generally follow the rule: the more common the characteristic, the greater the same-sex marriage support.

For example, split Australia's 150 electorates into buckets — more secular and less secular — and a pattern is obvious.

Or consider two buckets based on income: wealthier (where a greater proportion earn at least $1,500 per week) and less wealthy. Again, the pattern is clear.

For characteristics like religion it is easy to imagine why there might be a pattern.

For others like income, the relationship is less obvious.



Dr Perales said one way individuals deal with economic and other threats is via "displaced intergroup competition".

"Individuals with lower incomes may hold more negative attitudes towards sexual minorities and be less supportive of the rights of same-sex couples than their wealthier peers," he said.

The outliers

In Goldstein, in Melbourne's south-east, less than a quarter of residents are in their 20s and 30s. At 21 per cent, its rate is among the lowest in the country.

Yet more than three in four of its residents voted in favour of same-sex marriage — the ninth-highest such figure.

Goldstein MP Tim Wilson, himself part of a same-sex couple, said the low number of young adults was due to the absence of further education in the electorate, but it was more than made up for by high numbers of educated, often female, professionals.

His community is a place where "basic values around decency, respect, commitment, and responsibility resonate strongly", he said.

"They read newspapers, they watch television, news programs on television. They're interested in the world.

"And so I think its just a package deal of all the factors."

Goldstein also has an unusually low number of people who reported as being part of a same-sex couple, particularly given the area's high level of support for same-sex marriage.

On the other hand, Barton, in Sydney's west, has double Goldstein's rate for same-sex couples, even though support for same-sex marriage there was 43.6 per cent.

It is part of a band of western Sydney electorates where the majority of respondents were opposed to same-sex marriage.

ABC election analyst Antony Green linked this resistance towards same-sex marriage to "cultural differences", given this area is home to a larger proportion of people born in non-English-speaking countries.

What actually gives us our views

Dr Perales said using data to predict a group's attitudes towards social issues is more difficult than predicting other, more tangible outcomes, such as people's earnings.

And it is even harder to pull apart exactly why an individual thinks a certain way.

"While we can clearly discern those factors that, on the average, are powerful predictors of attitudes towards same-sex relationships, it is very hard, if at all possible, to predict individual opinions," he said.

Even mathematical models, according to his research, can predict only about one-fifth to one-quarter of variations in attitudes.

This unpredictability "could be attributable to factors for which there is no measurement in the data, for example, how many LGBTI friends or acquaintances the person has, or simply to each person's individual idiosyncrasies".

Mr Wilson believes people's views on same-sex marriage are affected by human contact.

"It's easy to change people's attitudes to marriage for same-sex couples because it's simply a journey out of people's interaction with other people," he told the ABC.

"If they have interaction with a friend or family member who the same-sex marriage debate directly affects, their attitude shifts very quickly."