The honesty problem
Most of it happens in rooms like this one
Not on a street, not near a school gate, and not by anyone a monitoring programme has ever heard of. A proximity alert cannot reach the majority of child sexual abuse, and a design that will not say so out loud gets dismantled by the first informed reader.
This page exists to stop the concept over-claiming. The number it produces is small. That is an argument for scoping the claim precisely — not for abandoning the work.
Who actually harms children
Of sexual assaults on victims under 18 that were reported to law enforcement:
- 34.2% of victims under 18 in sexual assaults reported to law enforcement were assaulted by a family member.Snyder, Sexual Assault of Young Children as Reported to Law Enforcement, Bureau of Justice Statistics NCJ 182990 (2000)
- 58.7% were assaulted by an acquaintance.Snyder, BJS NCJ 182990 (2000)
- 7.0% were assaulted by a stranger.Snyder, BJS NCJ 182990 (2000)
This is the share of cases REPORTED TO LAW ENFORCEMENT, which is not the same as the share of cases that occur.
And of the people committing these offences, 95.9% of sexual offence arrests over a 21-year period in New York were by people with no prior sex offence conviction.
Sandler, Freeman & Socia, Does a Watched Pot Boil?, Psychology, Public Policy, and Law 14(4):284–302 (2008)
One state, one period. It is the most-cited figure of its kind, not a national constant.
Those two facts together are the constraint. A proximity system can only ever see someone who is enrolled — which means convicted, and monitored. The overwhelming majority of people who sexually abuse a child have never been convicted of a sex offence, so they are not in the system to be seen at all.
Why family contact scores zero, not “less”
A proximity rule works by noticing that two people are near each other when they have no reason to be. A parent, step-parent, sibling or household member has every reason to be near the child continuously and lawfully. There is no threshold that separates abuse from ordinary family life, so the model scores this category at zero rather than at some small number. If it scored it at anything above zero it would be inventing capability.
Coverage model
Move any assumption. The arithmetic is the same arithmetic used above.—
of child sexual abuse cases reported to law enforcement could, in principle, produce an alert from this system.
The model multiplies four independent fractions and does nothing else. Its weakness is that the no-prior-conviction rate is taken from all sexual offence arrests, and the stranger-abduction subset plausibly has a higher prior-conviction rate than the general population of offenders. If that is true, the stranger figure here is an underestimate. The model cannot correct for it without a source, so it does not pretend to.
The right way to say it
A small share of cases, and the worst ones
Low frequency is not low value. The slice a proximity alert can reach is the slice with the worst outcomes.
about 115 stereotypical kidnappings — taken by a stranger or slight acquaintance and held overnight, transported a distance, ransomed, killed, or intended to be kept — were estimated in a year.Finkelhor, Hammer & Sedlak, NISMART-2, Office of Juvenile Justice and Delinquency Prevention (2002)
A 1999 estimate with wide confidence bounds. Later NISMART work put the figure in the same order of magnitude.
That is a very small number against the scale of child sexual abuse overall, and it is the category with the highest rate of serious injury and death. Systems aimed at rare, catastrophic events are ordinary in every other field of safety engineering. Nobody argues that because structural fires are rare, sprinklers are a poor investment.
So the claim to make is this one, and only this one:
The defensible claim
Safe Perimeter closes a specific gap: it detects sustained, unexplained physical proximity between a child and an adult under active monitoring, in the places a fixed geofence cannot see. It does not detect abuse by family members, by trusted acquaintances, or by anyone who has never been convicted — which together are the large majority of cases.
Stated that way, the concept survives a hostile reading. Stated as a solution to child abuse, it does not survive the first paragraph.
One more thing the physical layer does not touch
Grooming of older children — roughly twelve to seventeen — has largely moved online. It does not require physical proximity to begin, and by the time it does require proximity the relationship is usually already established and the meeting is consensual from the child’s point of view. A proximity alert has nothing to say about any of it.
This is worth naming explicitly rather than leaving as an unstated gap, because it is the most common form the harm now takes among teenagers, and because 2017 the Supreme Court struck down a ban on registrants accessing social media. — the online layer has its own constitutional constraints, and they are not the same ones.
Packingham v. North Carolina, 582 U.S. 98 (2017)