You don’t need to be a psychiatrist to know that the Highland Park shooter is sick in the head. 

His evil act is unfathomable, but he does fit a familiar pattern of mass killers: alienated young male stoners who appear to be in the grip of a distinctively American madness. 

Those who knew the 21-year-old suspect, Robert Crimo III, say he habitually smoked cannabis, a habit he appeared to share with young mass shooters, including at Uvalde, Dayton, Parkland and Aurora. 

Obviously weed didn’t make them commit their evil acts, but it may have scrambled their brains enough for empathy to take a holiday. 

As the country rushes headlong into the embrace of Big Weed, we need to heed the warning signs, not least in the scientific literature which increasingly shows that cannabis triggers psychosis, and in the emergency rooms where mentally ill kids are the living proof of its harms. 

The higher the potency of THC, the worse it is, especially for the developing adolescent brain. 

But virulent attacks always greet any hint of opposition to wholesale drug legalization. Youth mental illness is a crisis in this country and yet we are not allowed to discuss a scientifically verified ­trigger. 

So, let’s report what clues we have about Crimo’s state of mind, talking about “red flags.” 

He was reported to police in April 2019, when he was 18, after he threatened to take his own life, Highland Park Police said. That incident was dealt with as a “mental health” matter. 

Five months later family members again contacted police to say Crimo “was going to kill everyone.” Multiple knives were confiscated from his home. 

Two months later, when he was 19, police say his father, 58-year-old former deli owner Robert Crimo Jr., sponsored him for an FOID card — the license needed in Illinois to buy a gun. 

Attorney Steven Greenberg says the parents deny that their son was suicidal or threatened to kill anyone. Interviewed on Chicago cable network NewsNation, Greenberg refused to respond to questions about Crimo’s mental-health issues but said his family was being scapegoated for gun-control failures. 

“I don’t think anyone is ever aware of any red flags that make them think that their son is going to go out in their own community and start shooting people . . . These were wonderful parents.” 

Other reports paint a darker ­picture. 

Crimo’s mother, holistic practitioner Denise Pesina, 48, was filmed by ABC News Tuesday screaming at police outside her home and exposing her breast at them. 

In 2015, when Crimo was 14, she reportedly was charged with domestic battery over “a physical dispute that had occurred while driving” at 3.37 a.m. near the family home, in the affluent Chicago suburb of Highland Park, according to a police report posted on the website Patch. 

When Crimo was young his parents were “a problem,” his former coach Jeremy Cahnmann told Fox News Digital. “There wasn’t a lot of love in that family.” 

They were always the last to pick him and his brother up from an after-school Nerf football program. 

“Every week, the Crimos were the last kids there, and we’d have to call their parents to pick them up,” said Cahnmann. 

Pesina “got into it once with one of the heads of the program, she was yelling. It seemed like her kids were a nuisance to her.” 

At some point his father moved out of the family home into a house two miles away in Highwood, while Crimo stayed with his mother and the Highland Park house fell into disrepair. 

“It looks like it should be condemned,” a neighbor told Fox. 

About two years ago, around the time he lost his job at Panera Bread at the onset of the pandemic, Crimo moved into live with his father and uncle. 

Former friends describe the unemployed rapper by this time as smoking weed habitually. 

Nick Pacileo, 22, used adjectives such as “timid” and “quiet” to describe the boy he used to skateboard with from eighth through 10th grade. 

But when Crimo turned 18, his personality changed, Pacileo told NBC News, and he became depressed over a girl. 

“Instead of therapy he turned to drugs . . . 

“He definitely thought there was a border in the mind that needed to be broken through the mind. Very third-eye type of stuff that kind of goes along with the psychedelic rap and drugs.” 

Another former friend, Bennett Brizes, described the Crimo he knew from age 14 to 17, as “an isolated stoner who completely lost touch with reality.” 

Brizes, a college student in LA who said he used to “make music” with Crimo, posted a series of tweets and photos after the Fourth of July massacre describing his former friend as “lost.” 

More coverage on the Highland Park parade shooting

He also posted a screenshot of a Feb. 2, 2021, message he said was from Crimo: “Oi my mind is everywhere nowadays.” 

Cannabis doesn’t explain everything about Crimo and other mass shooters, but it deserves at least some debate amid the endless partisan bickering over gun laws — which already are among the country’s strictest in Chicago, the murder capital of ­America. 

The AR-15 used by Crimo is banned in the city and Illinois has a red-flag law designed to stop him buying guns after his disturbing contacts with police. 

But you need more than a new law on the books. You need to enforce it, and that’s something Democrats have made increasingly difficult. 

In any case, gun control is not the silver bullet. Something has gone wrong with America’s youth. 

US Surgeon General Vivek Murthy issued an urgent warning in December about the “devastating” youth mental-health crisis, which has been exacerbated by COVID lockdowns. 

We can’t address the crisis without considering the effect of teens’ cannabis use and the increased potency of the products they consume. 

The New York Times last month warned of the high potency of cannabis products in the newly deregulated legal market and the potentially harmful effects to young brains: “Psychosis, Addiction, Chronic Vomiting: As Weed Becomes More Potent, Teens Are Getting Sick”. 

THC, the active ingredient in cannabis, 20 years ago was at about 4% potency, but today’s Big Weed products are close to 100%. 

We have known for at least 15 years that cannabis use can increase the risk of psychosis in susceptible people by about 40%, according to the medical journal Lancet. 

A study last year of 204,000 people aged 10-24 in the Journal of the American Medical Association’s pediatrics publication found that cannabis use and abuse is associated with depression, bipolar disorder and increased risk of ­suicide. 

The one thing we should not have done was make it easier for young people to access such a potentially harmful drug. But that is the political climate heading to the midterms in November.