WASHINGTON — The two young men accused of carrying out the massacres in Buffalo and Uvalde followed a familiar path: They legally bought semiautomatic rifles right after turning 18, posted images intended to display their strength and menace — and then turned those weapons on innocent people.
As investigators and researchers determine how the tragedies unfolded, the age of the accused has emerged as a key factor in understanding how two teenagers became driven to acquire such deadly firepower and how it led them to mass shootings.
They fit in a critical age range — roughly 15 to 25 — that law enforcement officials, researchers and policy experts consider a hazardous crossroads for young men, a period when they are in the throes of developmental changes and societal pressures that can turn them toward violence in general, and, in the rarest cases, mass shootings.
Six of the nine deadliest mass shootings in the United States since 2018 were by people who were 21 or younger, representing a shift for mass casualty shootings, which before 2000 were most often initiated by men in their mid-20s, 30s and 40s.
“We see two clusters when it comes to mass shooters, people in their 40s who commit workplace type shootings, and a very big cluster of young people — 18, 19, 20, 21 — who seem to get caught up in the social contagion of killing,” said Jillian Peterson, a criminal justice professor who helped found the Violence Project, which maintains a comprehensive national database of mass shootings.
There is no single, easy explanation for why young men are more likely to engage in mass shootings. (Girls and women make up a small percentage of all perpetrators.) But many of the causes cited most often by law enforcement officials and academics seem intuitive — online bullying, the increasingly aggressive marketing of guns to boys, lax state gun laws and federal statutes that make it legal to buy a semiautomatic “long gun” at 18.
The shootings come against a backdrop of a worsening adolescent mental health crisis, one that predated the pandemic but has been intensified by it. Much of the despair among teenagers and young adults has been inwardly directed, with soaring rates of self-harm and suicide. In that sense, the perpetrators of mass shootings represent an extreme minority of young people, but one that nonetheless exemplifies broader trends of loneliness, hopelessness and the darker side of a culture saturated by social media and violent content.
In addition to Buffalo and Uvalde, Texas, there was a mass shooting at supermarket in Boulder, Colo., in March 2021 that the police said was carried out by a 21-year-old man; a massacre by what authorities said was a 21-year-old gunman targeting Hispanic shoppers at a Walmart in El Paso in August 2019 that resulted in 23 deaths; a school shooting in Santa Fe, Texas, in which a 17-year-old student is accused of killing eight students and two teachers in May 2018; and the killing of 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., in February 2018 by a 19-year-old former student.
Only two of the 30 deadliest mass shootings recorded from 1949 to 2017 involved gunmen younger than 21: The first was the massacre of 13 people by two teenagers at Columbine High School in 1999, and the second came when a 20-year-old killed 27 people, most of them children, at Sandy Hook Elementary School in Newtown, Conn., in 2012.
A shooting on Wednesday in Tulsa, Okla., in which a gunman killed four people and wounded several more before apparently taking his own life, defied the recent pattern. The police said they believed that the gunman, whom they had not identified, was between 35 and 40 years old.
Frank T. McAndrew, a Knox College psychology professor who studies mass shootings, said almost all of the young killers he has researched were motivated by a need to prove themselves.
“These are young guys who feel like losers, and they have an overwhelming drive to show everybody they are not on the bottom,” he said. “In the case of the Buffalo shooter, it was about trying to impress this community of racists he had cultivated online. In the case of the kid in Uvalde, it was about going back to the place where you felt disrespected and acting out violently.”
Ms. Peterson added: “And since Columbine, they have tended to study and emulate each other. It’s a growing problem.”
In almost every case, social media or interactive online game platforms played some role, mirroring the ubiquity of online youth culture over the past two decades.
In the late 1990s, at the dawn of the social media age, one of the gunmen at Columbine created a blog on AOL to detail his violent thoughts.
The 22-year-old college student who murdered six people in Santa Barbara, Calif., in 2014 offered one of the most direct expressions of a gunman’s mentality in a video posted on YouTube: The gun, he said, gave him a sense of power.
The Buffalo gunman, emulating the 28-year-old anti-Muslim terrorist who massacred 51 people in Christchurch, New Zealand, three years ago, live streamed himself as he methodically killed shoppers because they were Black. The man charged with the killings in Uvalde used Yubo, a relatively new platform, to share menacing messages in which he seemed to telegraph his plans.
“It’s a way for kids to flex,” said Titania Jordan, with Bark Technologies, an online safety company that monitors the use of platforms for violent content. “It’s a way for them to show strength if they are bullied, or left out. It’s just a part of the narrative now in all these cases — there’s always a social media component.”
There is also a biological one. Scientists have long known the teenage and post-teenage period is a critical time for brain development and a time, for most teenage boys, often characterized by aggressive and impulsive behavior. Girls of the same age, by contrast, have greater control over their impulses and emotions.
Overall, boys and young men account for half of all homicides involving guns, or any other weapon, nationwide, a percentage that has been steadily rising. Exactly 50 percent of all killings in 2020, the last year comprehensive data is available, were committed by assailants under 30, according to the F.B.I.’s uniform crime data tracking system.
Mass shootings, defined by most experts as involving the deaths of more than four people, are rare; shootings on the scale of Buffalo and Uvalde, with more than 10 victims, are even less common. Around 99 percent of all shootings in the country involve fewer victims, are the result of crime or personal disputes, and are motivated drug activity, gang conflict, domestic violence and personal disputes, according to statistics compiled by the federal government and academic.
“Why are a disproportionate number of crimes committed by males in their late teens and early 20s?” asked Laurence Steinberg, a professor of psychology and neuroscience at Temple University who has worked extensively on issues involving adolescent brain development.
The explanation, he said, includes the increasingly well-understood neurobiology of the teenage years. During adolescence, a “huge mismatch” develops between parts of the brain that cause impulsive behavior and emotional sensitivity and other parts of the brain that regulate acting out on such impulses, Dr. Steinberg said. Men, he added, tend typically to have an even higher, faster peak in arousal, while women see a higher peak in regulation at an earlier age — and therefore “at every age, males are more sensation-seeking.”
The height of that mismatch tends to be in the late teens or early 20s. “Then the regulatory systems start to catch up to the impulses, and you’ve got this gradual improvement in ability to control thoughts, emotions and behaviors ongoing into the early 20s,” Dr. Steinberg added.
The changes in brain development are accompanied by the disorienting societal passage from boy to man, with all the turbulence that entails even in healthy boys. There are “major differences in socialization for males and females related to aggressive behavior, appropriate ways to seek support, how to display emotions and acceptability of firearm use,” said Sara Johnson, a professor of pediatrics at the Johns Hopkins University School of Medicine.
Young men are “almost universally” in transition “in their relationships, living situations, lifestyles, education, occupation,” while “at the same time they have substantial autonomy from the adults in their lives and may find themselves negotiating with little support or supervision,” Dr. Johnson said.
Yet what differentiates mass killers from other young men who do not act on these impulses is hard to define, and even harder to counter: madness.
Still, the vast majority of young men with mental health disorders, even serious ones, never commit acts of violence. They are more likely to be victims, or impulsively hurt themselves, than to painstakingly plot violence against others.
Republicans, countering Democratic calls for tightened gun controls, have seized on improving school safety and upgrading mental health services after the recent massacres.
Conservatives are also resisting efforts by congressional Democrats to raise the legal age to buy a semiautomatic rifle from 18 to 21. A Republican-appointed federal judge recently struck down California’s attempt to increase the age. The state enlisted Dr. Steinberg and other experts to make the scientific case for keeping such weapons out of the hands of teenagers.
Their arguments did not prevail. “America would not exist without the heroism of the young adults who fought and died in our revolutionary army,” Judge Ryan Nelson, speaking for a two-to-one majority in the U.S. Court of Appeals for the Ninth Circuit, wrote in an opinion released on May 11, three days before the rampage in Buffalo.
With few policy safeguards, mental health professionals and local authorities have been left to spot and stop potential gunmen, with uneven success.
In 2018, the police arrested two boys, 13 and 14, after receiving a tip just before the anniversary of the Columbine shooting. The teenagers were planning to target a school in Uvalde and wanted to rob a neighbor’s house to obtain weapons. The suspect in the Uvalde massacre was not involved in that plot.
Over the years, Jill H. Rathus, a therapist in Great Neck, N.Y., has seen her share of young men who seemed to be a danger to themselves or others, including one whose mother feared her son would become a gunman. He did not.
Dr. Rathus and other experts cautioned that there were vast differences between suicidal and homicidal behavior, but she also said she saw some overlap in certain feelings that contributed to growing acts of violence directed at self and at others. “There’s an incredible sense of aching despair plus hopelessness, and then there’s a sense of a lack of meaningful connections,” Dr. Rathus said. “Then there’s access to lethal means, that’s the center.”
In 2006, Dr. McAndrew, the Knox College psychology professor, and two of his colleagues set out to test the effect of guns on the behavior of young men, monitoring the testosterone levels, and signs of aggression, in 30 male college students when they were given a children’s toy and an actual firearm.
“The presence of a gun changed their behavior significantly,” he said. “Just holding a gun gave you guts.”
Glenn Thrush reported from Washington, and Matt Richtel from San Francisco.