Although the primary aim of Just as Deadly is to present information on female serial killers (FSKs), the author discusses a comparison of FSKs and male serial killers (MSKs) to illustrate the dramatic differences between their crimes. As described in Evolutionary Behavioral Sciences, the author and her team investigated sex differences in serial murder through an evolutionary psychological lens but also collected data on other background and mental health variables, as empirical comparisons of FSKs and MSKs are virtually nonexistent in the scientific literature. Data collected from 55 MSKs and 55 FSKs revealed sex differences in relationship status, educational attainment, and socioeconomic status. Moreover, nearly 90% of MSKs and 43% of FSKs had reported mental health issues. These results show that MSKs have more than a fourfold chance of having a mental illness compared to the population and are twice as likely as FSKs to have documented issues. The cases of MSKs Ed Gein and John Wayne Gacy describe mental illness issues and are used to demonstrate drastic differences in serial murder modus operandi and victims.