An expert, in this article from City Journal, examines the evidence.

An excerpt.

The Major Cities Chiefs Association recently issued its crime figures for the first half of 2016, and some of the numbers are scary. In four out of five violent crime categories, including murder, the most trustworthy measure, this year is starting off worse than last—and 2015 was more violent than the preceding year. Six out of the ten biggest American cities have seen double-digit increases in 2016; overall, crime is up 14.8 percent. The good news is that murder declined 6.4 percent in New York City and Houston saw a 51.4 percent drop.

Only a pollyanna would pooh-pooh the chiefs’ negative figures and dismiss talk of a new crime rise. Clearly, crime is up, but there’s a big difference between a crime spike or surge, which lasts a year or two, and a true crime boom or wave, which could run for decades. I define a crime wave as a period of sustained high crime, manifested by homicide victimization rates of eight or more per 100,000. Rates over the past few years have been hovering around 5 per 100,000. The last wave, which began at the end of the 1960s and ended in the middle 1990s, ran for two-and-a-half decades, as did the earlier twentieth-century crime boom, which dragged on from 1910 to 1936.

What we really want to know is whether we’re headed for another crime wave, like the devastating crime tsunami of late memory. No one can be sure of the answer, of course, since the future is the greatest crime mystery of all, but if we look at the factors associated with previous booms and apply them to our current situation, we may get a better idea.

Over the course of U.S. history, four factors have been especially significant in explaining crime booms. The first is migrations within or to the United States of groups with so-called “honor cultures.” Previous crime escalations were spurred by immigrations and migrations to major urban areas or to discrete rural locales of groups that engage in a disproportionate use of violence to resolve interpersonal conflicts. Examples are the famine Irish of the mid-nineteenth century, southern Italian immigrants at the turn of the twentieth century, the Mexican migration of the 1920s, and the great black migrations of the 1920s and the post-World War II era. Each of these migrations increased crime at their destinations and raised total rates nationwide.

In the 1960s, for example, over 800,000 blacks left the South for urban settings in the North and West. In the succeeding decade, another 1.5 million relocated—the biggest decadal migration of African-Americans in American history. The black population in the Northeast nearly doubled, rising 93 percent between 1950 and 1970. Chicago, for example, went from 23 percent African American in 1960 to 40 percent in 1980.

Over a 20-year span, from 1976 through 1995, African Americans committed a majority of the criminal homicides in the United States—53.2 percent, to be precise. This is quite extraordinary, given that during this period, blacks comprised around 12 percent of the U.S. population. But this excessive black murder rate was not a new development. African-American homicide had been exceptionally high at least since the late 1880s. In the 1920s, black homicide rates were, on average, seven times those of whites. From 1976 through 1995, they were eight times the white rate. Although racists attributed these rates to biological factors, the real explanation is the honor culture developed in Dixie and first found among Southern whites.

The second key factor is demographic change that creates an outsize youth population. Young people, especially males, are responsible for the overwhelming majority of violence. A sudden and major increase in the size of this population, such as occurred in the late 1960s, is a big risk factor for violent crime.

The young male population jumped 29 percent in the 1960s and 43 percent in the fol­lowing decade. One study found that between 1958 and 1969, age composition alone accounted for 45 percent of the increase in all crime and 11 percent of the growth in violent crime. Economist Steven Levitt, examining the years between 1960 and 1980, attributed 22 percent of the rise in violent-crime rates to changes in age structure.

The increased youth population also produced “contagions,” in which behaviors multiply rapidly as a consequence of the tendency of young people to copy one another. These behaviors may be socially positive (enrolling in college), neutral (getting tattoos), or negative (engaging in crime). If the behavior reaches a critical stage or “tipping point,” it increases explosively. This is precisely what happened with crime in the late sixties.

Significant weaknesses in the machinery of law enforcement, especially in the destinations of the honor cultures, are another contributing factor. When police, courts, and jails/prisons are unable to cope with crime, their failures serve as an incentive to increased lawbreaking. This occurred in the Wild West and in the rural South in the nineteenth century, in the big Northeast cities in the mid-nineteenth century (leading to the creation of urban police departments), and again in the late 1960s.

When the great crime tsunami engulfed the country, the criminal justice system buckled under the strain. Police couldn’t cope with the sudden increase in offenders, courts couldn’t con­vict or imprison as many defendants as they had earlier, and people who were convicted spent less time behind bars. Police clearance rates (roughly, the ratio of arrests to reported crimes) tumbled in the late sixties. In 1950 and 1960, around four in ten reported robberies were cleared by police. That declined to under three in ten by 1970. And by 1980 and 1990, only one in four robberies was solved.

And not only were fewer criminals incarcerated; they also served less time once committed. For every 1,000 arrests for serious crimes, 299 offenders went to prison in 1960. One decade later, only 170 were imprisoned—a drop of 43 percent. The time served in prison tumbled, too. Murderers had been locked up for a median 52 months in 1960, but only 42 months in 1970, a 19 percent decline.

Finally, the presence of male gangs, especially those engaged in distribution of illegal substances—whether alcohol in the 1920s and 1930s or cocaine in the late 1980s and early 1990s—has also played a key role. Gangs normally fight one another, of course. But gang rivalries may also serve acquisitive motives, such as when they compete for territory to control the profit from distributing proscribed substances, as occurred with alcohol during Prohibition and cocaine in the late 1980s. These rivalries were more intense than the usual gang fights. They also were more lethal because of the use of firearms, which put even innocent bystanders at risk.

Three of the factors described above were present in the late 1960s crime wave. The fourth factor, drug gangs, was responsible for restarting the crime tsunami after it had begun to abate in the early eighties.

Do these four factors suggest that we’re on the cusp of a new crime wave? Crime booms of the past have been cyclical and lasted about 25 years, which would mean that we’re due for one to start some time between 2015 and 2020. So perhaps the current surge is the start of a new boom. But the key factors point in a different direction.