Information on education came from Statistics Denmark and was operationalised as educational status according to the International Standard Classification of Education (Institute For Statistics, 2006) and length of education. Educational status was categorised into primary/lower secondary (primary) and upper secondary/tertiary education (secondary/tertiary) while length of education was included as a quantitative variable measured in years. As parity and AFB are known to be strong determinants of BC and strongly correlated with education, we included these variables in a subanalysis to investigate the degree to which these factors might be driving the association between education and BC. Parity was constructed as a time-dependent variable (0, 1, 2 þ births), while AFB was categorised as follows: no children, o26 years, 26 – 30 years, 31 – 35 years, 435 years. Owing to collinearity between the two variables, they were included in the model as a combined variable. The Danish Cancer Registry contains information on all primary incident cancers in Denmark since 1943, except non-melanoma skin cancer. Reporting is mandatory and registered information comes from a cross-checking of different sources. Cases of BC were indentified through ICD-7 (1980 – 1993) and ICD-10 (1994 – 2006) codes (170 and C50, respectively). Data analysis Using Cox regression, person years of follow-up were accumulated from baseline (age at 1 January 1980 or age 30 years) to the age at the end of follow-up (age at 31 December 2006), diagnosis of BC, death, or emigration, whichever was earliest. The analyses were adjusted for calendar time. Unpaired analyses were compared with intrapair analyses. The intrapair analyses were carried out by stratifying on ‘pair-number’ (Holt and Prentice, 1974), which permits the baseline hazard to vary freely between pairs while at the same time fixing it within pairs. The hazard function for twin j in pair i is given by hij(t,z) ¼ h0i(t) exp(bzij), where h0i(t) is the pair-specific baseline hazard and b is the effect of education that applies to all subjects. Differences in genetic relatedness between MZ (genetically identical) and DZ twins (share on average 50% of segregating genes) could further be exploited to make inferences about genetic and environmental confounding, respectively; assuming an effect of education in the unpaired analyses, genetic confounding would be indicated if the intrapair analyses showed a partial attenuation of the association in DZSS twins and a full attenuation in the MZ twins. Similarly, confounding from shared environment would be likely to account for the association if a full attenuation were observed in both DZSS and MZ twins. Finally, persistence of the association would be expected if education had an independent effect on BC beyond the effect of shared familial factors. Hazard ratios (HRs) were calculated and 95% confidence limits were corrected for interdependence of observations within twin pairs. In the intrapair analyses, a strata option was included (StataCorp. 2009, Stata Statistical Software: Release 11, College Station, TX, USA and StataCorp LP, College Station, TX, USA). Potential interaction between education and zygosity was examined. Association of education with BC within twin pairs? M Madsen et al Table 1 Descriptives on educational status and length of education (mean number of years) according to zygosity in a population of Danish female twins (N ¼ 16 310) 521 DZSS (N ¼ 10 042) Educational statusa Primary Secondary/tertiary Dicordance on education Binary definitionb Continuous definitionc MZ (N ¼ 6268) Educational statusa Primary Secondary/tertiary Dicordance on education Binary definitionb Continuous definitionc N 4395 5647 2706 6114 2269 3999 1174 3116 % 44 56 27 61 36 64 19 50 Number of years Mean s.d. Min Max 8.0 13.6 4.7 3.1 8.2 13.7 4.2 2.7 1.3 1.5 1.7 2.0 1.3 1.6 1.7 1.8 7 10.5 0.5 0.1 6 10.5 0.5 0.1 10 20 10 10 10 20 11 11 Abbreviations: DZSS ¼ dizygotic same sex twins; MZ ¼ Monozygotic twins. aMea- sured according to ISCED (International Standard Classification of Education). bEducation discordant twins where one twin has a primary education and the co-twin has a secondary/tertiary education. cEducation discordant twins that differ in their length of education measured in years, that is, 40 years difference. The unpaired analysis of risk according to educational status (Table 2) shows a statistically significant increased risk associated with a secondary/tertiary education compared with primary education (HR ¼ 1.39, 95% confidence interval (CI) ¼ 1.15 – 1.67) for all twins. Similar results were seen when DZSS and MZ twins were analysed separately, and an analysis of potential interaction showed no evidence of modification of educational effect by zygosity (P ¼ 0.66). For the length of education, we observed a 4% increased risk of BC associated with one additional year of education (HR ¼ 1.04, 95% CI ¼ 1.01 – 1.07). The same pattern was seen when stratified on zygosity. The intrapair analysis of all twins showed an association with educational status of the same magnitude as in the unpaired analyses (HR ¼ 1.38), although no longer statistically significant (95% CI ¼ 0.95 – 2.01). When the analysis was split on zygosity, the association was attenuated in the MZ twins (HR ¼ 0.82, 95% CI ¼ 0.41 – 1.67). However, CIs were wide due to the limited number of events. For DZSS twins, the association persisted (HR ¼ 1.70, 95% CI ¼ 1.08 – 2.69). Similar trends were suggested in the analyses by length of education. No statistically significant interaction between education and zygosity was found in the intrapair analysis (P ¼ 0.35). When the analysis was adjusted for parity and AFB, the effect of education changed only negligibly (educational status: HR ¼ 1.37, 95% CI ¼ 1.14 – 1.65 and educational length: HR ¼ 1.04, 95% CI ¼ 1.01 – 1.07) and the effect of the combined covariate was statistically insignificant (P ¼ 0.61). RESULTS In the cohort of 16 310 female twins, 518 developed BC during the follow-up period 1980 – 2006. As expected, more DZSS than MZ twins were discordant on education (Table 1). More twins were discordant by the quantitative definition, which expresses the difference in years of education within a twin pair. A larger mean difference in length of education was also observed among the DZSS twins. Additionally, a higher proportion of DZSS (44%) twins had a primary education compared with MZ twins (36%). DISCUSSION In this study, we examined the association between education and BC risk in Danish female twins with and without adjusting for familial background factors. In the unpaired analyses we found the expected increased risk associated with a secondary/tertiary compared with a primary educational status of similar magnitude to that previously reported (Kongevinas et al, 1997; Carlsen et al, 2008). This association was replicated in the intrapair analyses for all twins, suggesting that the & 2011 Cancer Research UK British Journal of Cancer (2011) 104(3), 520 – 523 y g o l o i e d i p E