Income High Medium Low Cohort 1981-86 1986-91 1991-96 1996-01 2001-04 % Change P (Trend) Pooled 1981-86 1986-91 1991-96 1996-01 2001-04 % Change P (Trend) Pooled 1981-86 1986-91 1991-96 1996-01 2001-04 % Change P (Trend) Pooled SR 95% CI 122 (111 - 134) 158 (147 - 170) 141 (133 - 150) 177 (169 - 186) 188 (179 - 197) 54% 0.04 156 (151 - 160) 122 (112 - 132) 149 (141 - 158) 152 (144 - 159) 165 (157 - 172) 181 (171 - 190) 48% < .01 152 (148 - 156) 114 (107 - 121) 126 (119 - 133) 139 (132 - 147) 162 (155 - 170) 167 (159 - 174) 47% < .01 140 (137 - 143) Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 Page 7 of 10 Table 4 Age and ethnicity standardised breast cancer rates, rate ratios (SRR), and rate differences (SRD) by income, age 25+ SRR 95% CI SRD 95% CI *p test is test for linear trend differences in incidence. Moreover, such genetic differ- ences will not account for rapid changes in rates over time in any group, and could only account for diverging ethnic group trends over time in the presence of some environmental exposure that is also changing over time and interacting with a varying genetic predisposition by ethnicity. Indigenous Hawaiian women also have surprisingly high rates of breast cancer given their distribution of risk factors, and it has been postulated that genetic poly- morphisms in the sex steroid and gonadotropin metabo- lism pathways, causing high endogenous hormone levels, are responsible for their high rates [58]. Dietary factors and high insulin-like growth factor levels are also mentioned as possible factors [57]. Indigenous Hawai- ians also have higher breast density than European Hawaiians [58]. It is possible that there may be similari- ties between the drivers of breast cancer rates in Māori and indigenous Hawaiian women, but more research is needed to elucidate potential mechanisms in both groups. However it is not obvious why Māori and Hawaiian women would have such different breast cancer rates from other groups of Polynesian women from the Pacific. The above discussion has been orientated mainly at trying to explain differences between ethnic groups’ rates on average, but our study also finds a more rapidly increasing Māori breast cancer rate than European/ Other. (The Asian rate also increased rapidly, but was prone to greater statistical imprecision - hence we focus on just Māori and European/Other trends.) This diver- gence over time may be due to changes in the factors which are driving the underlying differences between Māori and European/Other rates (although as noted above these factors are currently unknown). Alterna- tively, this change may be due to changes in the distri- bution of established risk factors amongst the Māori population, overlying the already higher risk for Māori. For example there is some evidence that Māori fertility rates are dropping more quickly than among European women, with a convergence on similar fertility patterns [37,59]. Rapid changes in postmenopausal obesity rates 1 1 1 1 1 0.99 (0.88 - 1.13) 0.94 (0.86 - 1.03) 1.07 (0.99 - 1.16) 0.93 (0.87 - 0.99) 0.96 (0.90 - 1.03) 0.67 0.98 (0.94 - 1.01) 0.93 (0.83 - 1.04) 0.80 (0.73 - 0.87) 0.99 (0.91 - 1.07) 0.91 (0.86 - 0.98) 0.89 (0.83 - 0.95) 0.94 0.90 (0.87 - 0.93) 0 0 0 0 0 -0.7 (-16 - 14) -8.8 (-23 - 5.4) 10 (-1.3 - 22) -13 (-25 - -1.2) -7.5 (-20 - 5.4) 0.59 -3.7 (-9.7 - 2.2) -8.8 (-22 - 4.6) -32 (-46 - -19) -1.9 (-13 - 9.0) -15 (-27 - -3.6) -22 (-34 - -9.5) 0.83 -16 (-21 - -10)