The use of default assumptions in risk assessment originated in unusual conditions. In the early 1970s, President Richard M. Nixon had declared a war on cancer. The U.S. Environmental Protection Agency (EPA) was a new agency created to deal with environmental hazards. While cancer was a well-known problem, scientific understanding of its causes were limited, and experimental methods were in an early stage of development. Although cancer was known to represent perhaps as many as 200 diseases involving disturbances in cell growth and death, some scientists proposed to reduce existing knowledge regarding causation to simplified generic statements referred to as "principles" applicable to the entire broad class of cancer.

In its 1976 Interim Procedures and Guidelines, EPA adopted the concept of "General Principles Concerning the Assessment of Carcinogenic Data."¹ The interim guidelines created a Cancer Assessment Group to review the data and provide advice on risk assessment. It was in that atmosphere that the default assumptions were developed. Scientific understanding of carcinogenesis has advanced significantly since then, but EPA continues to use the 1970s generic default assumptions. Nevertheless, EPA has sent some mixed signals about continuing their use.