been considerable, and through their influence the elasticity of the entire field of population research has been renewed.

The scientific rationale for including biomarkers in HRS is not fundamentally different from the rationale for including them in any population survey concerned with health. They validate and add nuance to self-reports of health, they allow richer modeling of pathways of influence between the socioeconomic and the physical, and they may capture aspects of health unknown to survey participants. This chapter gives examples of how each of these are realized in the HRS. The development of biomarker data in other studies of older populations in the United States, such as the National Survey of Midlife Development in the United States (MIDUS) and the National Social Life, Health, and Aging Project (NSHAP), and outside the United States in the English Longitudinal Study of Ageing (ELSA) and the Mexican Health and Aging Study (MHAS), has both provided models of what can be done and created great potential for comparative work with the addition of such data to the HRS.

Because of the unique place of the HRS in population surveys of aging, however, the decision to add biology to the HRS involved a number of other considerations, several of which were clearly anticipated by Weinstein and Willis in their chapter of Cells and Surveys (Weinstein and Willis, 2001). The HRS is a large longitudinal study that serves a large constituency of researchers from many different disciplines. At last count, there were over 6,000 registered users of the data, and over 1,000 unique authors of written research using the data. Putting its traditional aims at risk through attrition of respondents or elimination of critical established content would have been unacceptable. Similarly, the confidentiality of respondents had to be protected, as well as the integrity of a longitudinal observation study not be transformed into an intervention study.

The ethical issues were considered carefully by the HRS investigators as well as the institutional review board (IRB) governing the study. Notifying respondents of the results of well-established and commonly available diagnostic tests was deemed an ethical responsibility that overrides any concern that the information might alter future behavior. Because the tests contemplated by HRS assess familiar risk factors and do not identify, for example, life-threatening cancers, the ethical conflict is not particularly difficult at this time. Biological material stored in repository for future use is governed by a separate IRB review. Respondents were asked to consent to having this material stored anonymously for future research. Ethical issues arising from any particular future test will need to be addressed at that time. For example, it is conceivable that some test of scientific value might not be permitted if it carried with it the ethical obligation to notify children or other nonparticipants of the possibility of an inherited disease,