Demographic interest in fertility is not now driven by the biological imperative to have children for the survival of the group, but by the rapid population growth and the issue of unplanned fertility. The confluence of the largely successful worldwide effort to tame high fertility and the emergence of genetic approaches to understanding human behaviour provide a stimulus to review the focus of demographic work on fertility and to enlarge its concerns with biological and evolutionary questions. The NRC took on this task with the understanding that researchers in the area were pursuing a variety of independent approaches that required a common unifying focus. Some work in the area, such as studies of twins and adoptees, generated controversy despite being pursued aggressively. Offspring is the result of a study to examine these issues and seeks to encourage more balance and depth in the treatment of fertility in population studies.