Summary Using age-adjusted incidence rates and proportional incidence ratios, the risks of prostate cancer and breast cancer in three racial/ethnic groups- Spanish-surnamed whites, other whites and Japanese- were studied in Los Angeles County native residents and compared with those in immigrants and representative 'homeland ' populations. An algorithm based on social security numbers was developed and utilised to estimate age at immigration for non-US-born Los Angeles County cancer patients. For prostate cancer, the incidence rates in Los Angeles County were much higher than those in the homelands for each racial/ethnic group. However, prostate cancer rates of immigrants were similar to those of US-born patients in the Spanishsurnamed white and Japanese populations, regardless of age at immigration. For breast cancer, the incidence rates in Los Angeles County were also high compared with those in the homelands. However, the timing of immigration to the US was important in determining breast cancer risk. When social security numbers indicated that migration occurred later in life, rates for breast cancer were substantially lower than when migration occurred early, although they were still much higher than in the homeland populations. These findings suggest that environmental factors in early life rather than in later life are important in the etiology of breast cancer and that later life events can substantially impact the likelihood of developing clinically detectable prostate cancer.

In human populations, variation in mate availability has been linked to various biological and social outcomes, but the possible effect of mate availability on health or survival has not been studied. Unbalanced sex ratios are a concern in many parts of the world, and their implications for the health and survival of the constituent individuals warrant careful investigation. We indexed mate availability with contextual sex ratios and investigated the hypothesis that the sex ratio at sexual maturity might be associated with long-term survival for men. Using two unique data sets of 7,683,462 and 4,183 men who were followed for more than 50 years, we found that men who reached their sexual maturity in an environment with higher sex ratios (i.e., higher proportions of reproductively ready men) appeared to suffer higher long-term mortality risks than those in an environment with lower sex ratios. Mate availability at sexual maturity may be linked via several biological and social mechanisms to long-term survival in men. I n human populations, variation in mate availability, as indexed by sex ratios, has been linked to various social outcomes, such as marriage market performance, family formation, labor supply, and prevailing gender roles (Angrist 2002; Fossett and Kiecolt 1993;

Block et al. (I) reported a method to estimate year of birth using the Social Security number in cohort studies in which the age of a significant portion of cases was unknown. Social Security numbers have proven useful to us in a different way in a long-term follow-up study of childhood lead poisoning. We have been following cases diagnosed up to 68 years ago, and for many of these cases it has been decades since their vital status was estab-lished. We were able to obtain a Social Security number for most of these individuals but had difficulty ascertaining their vital status at any point after they were lost to medical follow-up. By using the tables and extrapolations provided by Block et al., we identified year and state of issue of each Social Security number. We knew, therefore, cases to be alive up to the date of issue, and we knew in which state to concentrate follow-up efforts. This method increased follow-up for 60 (13 percent) of the cases. On average, 14 years of follow-up were gained for each of the 60 cases, and total years of follow-up were increased 6 percent for the study group overall. The greatest gains in years of follow-up were for female cases who did not obtain Social Security numbers until middle age. This application of Social Security numbers may be of use to other investigators involved in retrospective follow-up studies, par-ticularly studies of childhood diseases where age of onset occurs prior to the age of obtaining a