The results showed that more than 93 per cent of respondents believed their income to be in the middle 60 per cent of the distribution. Most of us like to imagine we're middle income-earners. Ask us where we fit and almost no one admits to being either rich or poor (unless they're accused of not using much petrol). A survey conducted this year for the Australia Institute, found a similar result but put it a different way: nearly all Australians think the average income is the same as their own income. Of those respondents reporting their own annual household income to be between $20,000 and $40,000, 58 per cent believed the average income of all households lay within that range. For those on $40,000 to $60,000, 71 per cent thought this was average. For $60,000 to $80,000 it was 61 per cent, for $80,000 to $100,000 it was 55 per cent, and for $100,000 to $150,000 it was 51 per cent.

Even for those on more than $150,000 a year, a third of them thought that was average. According to the Australian Bureau of Statistics, the average household income in Australia is $80,700. But how could so many of us be so out of touch? How could we be so unaware of how the other half lives and which half we fit into? I'm sure there are various reasons, but one of the big ones is something that's been going on for years without most of us noticing. Our cities are becoming more socially stratified, with the better-off congregating down one end and the less well-off down the other. How could we be so unaware of how the other half lives and which half we fit into? These days, you're less and less likely to find suburbs with a cross-section of high and low income-earners, or highly and lowly educated people.

So we don't know how the other half lives because they are in the other half - the half we live far away from and rarely visit or even drive past. Pretty much all our family, friends and workmates are in the same half we're in. A study conducted last year by Jane-Frances Kelly and Peter Mares, of the Grattan Institute, Productive Cities, looked at maps of who lives where in Australia's largest cities and tracked how this had changed in the 20 years between 1991 and 2011. The authors found that the residents of our four biggest cities have enjoyed rising incomes and have become much better qualified. At the same time, however, our cities had become more polarised. "Increasingly, high-income residents with university-level qualifications cluster in suburbs close to city centres, while residents on lower incomes, and residents with vocational [trade certificate and diploma] qualifications, are more likely to live around the city fringes," they say. "In each city, it is also possible to identify particular areas of disadvantage, where a high proportion of residents have no formal qualifications beyond secondary school, where labour market participation is low and where a high proportion of young people are 'disconnected' - that is, neither working nor engaged in education or training."

In Sydney and Melbourne, individuals on higher incomes are clustered in inner suburbs and suburbs with desirable natural attributes such as beaches, trees and hills. In Sydney, the highest median incomes are found in inner, northern and harbour-side and beach-side suburbs, while the lowest median incomes are concentrated in western and south-western suburbs more distant from the CBD. In Melbourne, the highest median incomes are found in inner, eastern and bay-side suburbs, while the lowest median incomes are concentrated in more distant western, northern and south-eastern suburbs. A map also shows a clear pattern in house prices. "The premium placed on proximity to the city centre is evident in steep house-price gradients in Sydney, Melbourne, Brisbane and Perth," they say. Research by the Reserve Bank shows that, if you rank house prices for any of those cities according to their distance from the central business district, you get an almost perfect curve that (using figures from 2010) starts well above $1 million in Melbourne and Sydney and then declines steadily to about $300,000 when you're more than 60 kilometres from the centre.

This relationship between proximity and house prices has strengthened in recent decades, with average annual growth in house prices about 2 per cent higher in inner suburbs within five kilometres of the centre than on city fringes. Those people out in the boondocks have no idea how much we struggle with our mortgages - and we have no idea that they have problems, too. Price of petrol, for starters. Ross Gittins is economics editor