The number of perpetrators included in a registry would depend on how many states participate and what threshold is used to determine which individuals identified in state registries would be listed on a national registry. If a national registry included all persons identified as perpetrators in substantiated or indicated child maltreatment investigations within the past five or more years, it would initially include at least several million individuals, with an additional 500,000 perpetrators identified each year by states. To the extent restrictions were placed on who would be listed in a registry and for how long, numbers would be reduced. For instance, if a federal registry were to require that perpetrators be listed on a registry only if the standard of proof used to substantiate the case was at least as strong as "preponderance of the evidence," (that is, the investigation determined it is more likely than not that the maltreatment occurred) numbers would be significantly smaller, since many states use lower standards of proof for substantiating child maltreatment investigations. Figures would be smaller still if the registry included only persons for whom a civil or criminal judicial determination (rather than the child protective service agency's administrative determination) resulted in the perpetrator designation. Other possible limitations could exclude from the registry juvenile perpetrators (in the same way juveniles are excluded from most criminal justice databases) or those whose perpetrator designation is under appeal. Any of these limitations would affect a registry's size.
Another factor that may affect the number of individuals identified as perpetrators is the implementation of differential response systems in child protective services agencies. Differential response is an approach to child protection activities in which only the most serious child maltreatment cases are investigated and lead to a perpetrator designation. Less serious cases, often the vast majority of all cases, are instead routed to an alternative response that typically involves a family assessment and the provision of supportive services. When differential response systems are implemented, the number of individuals labeled as perpetrators typically drops dramatically. As of 2010, 13 states had implemented differential response systems state wide and additional states had implemented pilot sites to test the approach. To the extent these systems are extended, a national registry of child maltreatment perpetrators would include fewer names and could be expected to identify fewer interstate perpetrators. On the other hand, those named as perpetrators in jurisdictions operating alternative response systems could be expected to present the most serious threats to children. Identifying such individuals may have greater safety benefits than identifying a wider range of individuals who have come into contact with child protection agencies. Such serious cases are also more likely to involve criminal charges, however, and may therefore be cases more likely to be revealed using existing processes for conducting criminal background checks.