## 4 Discussion  
Studies of the associations of sociodemographic factors with depression, especially among the Australian population, are minimal, and the results are non-conclusive. The current study aimed to investigate the association of sociodemographic factors with depression from the 2007 National Survey of Mental Health and Wellbeing (SMHWB). In this study, we reported the prevalence and association of a number of sociodemographic factors with lifetime depression and 12-month depression. The current study shows that the prevalence of lifetime and 12-month depression was higher in females and in younger people. Household income was another significant predictor of depression, especially in females. The results for gender were consistent with previous studies that females had significantly higher rates of depression than males [5; 8; 9; 10; 16], and depression is more prevalent among young women than young men [28]. Higher rates of depression among females can be due to several reasons, including reproductive and environmental factors, behavioral factors and could be genetics, and response bias [16; 29]. However, some studies conducted in Australia [20; 21; 22] did not find gender as a significant factor for depression. This may be due to some of these studies using a different diagnostic tool for depression such as HADS-D [21], and the creation of an index to measure depression [22]. This study had a greater power due to the number of participants due to it being a national study. The association of older age with a lower prevalence of depression aligns with some studies [5] and contradicts some other studies [6; 7] outside Australia. However, in some Australian studies [20; 21; 22], age was not a significant factor in depression. The increasing rate of depression among younger people may be due to several reasons, including the stigma toward people with depression decreasing [30]. Over time, an increase in reporting of mental health problems among younger people could be another reason [31]. Other behavioral factors and life events during early adulthood could be another factor behind the increased prevalence of depression among younger people [32]. Other potential factors related to age that are not included in this study can explain the decline in depression rates in older age. Future studies have the potential to better understand the association of the older generation with a lower prevalence of depression. Level of education was found to be insignificant in this study, which is not in agreement with the international studies or the available Australian study [23]. No association could be attributed to the categories of education levels chosen in the current study. Other studies defined broader categories such as \"primary school\" to \"college/university \(\geq\)4 years\" [6], and \"high school only\" to \"bachelor's degree or above\" [26]. In the current study, the level of education was categorized very narrowly, such as year nine, year ten and so on. We attempted to combine different categories; however, it was abandoned due to the difficulty of overlapping categories when combining the secondary and tertiary types. A significant association could be if this study followed similar categories, especially if no education could be an independent group. Income was found to be significant in this study. The fifth quintile of income was associated with a lower prevalence of lifetime and 12-month depression. This finding was consistent with previous findings [12; 13; 22]. The association of higher income with a lower prevalence of depression could be explained by the mechanisms of social causation and social selection theories that have been posited in understanding the link between mental health and income [33]. Social causation sets that adversity, stress, and reduced capacity to cope related to low-income increase the risk of development of mental illness [34; 35]. However, more factors are needed to explain the living conditions in addition to the household income.