This week, FBI Director James B. Comey became the latest public figure to claim that national scrutiny of police has contributed to a "spike" in violent crime in America. This nefarious theory, dubbed the "Ferguson Effect" has become a regular talking point on conservative media and has been endorsed by well-respected publications such as the New York Times. This theory, as many experts have noted, relies on assumptions and not facts. Three assumptions form the foundation of the "Ferguson Effect":
1. Police have become less aggressive post-Ferguson because they face increased scrutiny and don't want to become the next "viral video"
2. Violent crime has increased nationwide after Ferguson
3. The increase in violent crime is due, at least in part, to this change in police behavior
FBI Director Comey himself has conceded there is no evidence to support the "Ferguson Effect."
There is, however, evidence to debunk it.
Let's examine the first premise, essentially that police are becoming "less aggressive" because they don't want to be the next officer to be scrutinized for inappropriate use of force. If this were true, we would expect police to be hesitant to use force, especially deadly force against unarmed black civilians (which have resulted in the most scrutiny). Unfortunately, Mr. Comey's agency still hasn't managed to collect comprehensive data on police use of force. Crowd-sourced efforts, on the other hand, have collected data to test this assumption, and the data do not support the notion that police are becoming "less aggressive." 
According to the Killed by Police database, police killed 665 people nationwide in 2014 before the Ferguson protests began on August 9th. In 2015, police killed 721 people over the same time period -- an 8% increase. Moreover, police killed more black people including more unarmed black people during this period in 2015 compared to the same period in 2014. In what world does this constitute a "less aggressive" form of policing?
The second premise of the "Ferguson Effect" references a spike in violent crime across the nation. Again, the facts do not support this claim. Analyses by the Marshall Project and the Justice Department have found no evidence that violent crime has significantly increased among the nation's largest cities. In fact, 2015 has been one of the safest years in the past two decades. Proponents of the "Ferguson Effect" may counter that crime has increased in cities that have experienced substantial unrest such as St. Louis. While crime has increased in St. Louis, this rise was already underway before the Ferguson protests. Meanwhile, a number of other cities that have experienced sustained protests such as Washington DC, Oakland, and Seattle have seen crime decrease or remain level. As such, fears of a national "crime spike" are not based in reality.
The third premise of the "Ferguson Effect" suggests that police restraint causes crime to increase. There is no reason to assume a discredited phenomenon (i.e. less aggressive policing) is causing a nonexistent crime spike. But even if we assume that police are generally behaving less aggressively (except when they decide to use deadly force), it's not clear why this would lead to increased crime unless you assume that aggressive police officers are the solution to keeping our communities safe. If police can't do their jobs without violating the constitutional rights of black people, then we must question the institution of policing rather than the protesters who expose its transgressions.
In the end, the "Ferguson Effect" lacks factual basis. It took months of nationwide unrest, a litany of shocking videos and detailed reports of police violence to convince the nation that policing in America needed to be fundamentally changed. The fact that a theory lacking evidentiary support could be so hastily endorsed by some of the nation's foremost institutions speaks to the enduring power of the belief that aggressive policing is the only way to keep black communities safe. This notion, applied exclusively to black communities, is exactly what needs to change.
 For review of these crowd-sourced databases, see here.
 While other forms of police aggression would add greater depth to such an analysis, national data on non-fatal police use of force incidents is currently unavailable.
A follow-up analysis coding for armed/unarmed status over this period finds 188 black people, 59 of whom were unarmed killed from 1/1/14 to 8/8/14 compared to 200 black people, 67 of whom were unarmed that were killed by police over the same period of 2015. You can view these data here.