When it comes to getting high on marijuana, the conventional wisdom is that one of two species of the plant—cannabis sativa or cannabis indica—will determine whether you get an up high or you get couch lock. (A third species, cannabis ruderalis, has also been identified, but it doesn’t have much psychoactive effect and is more hemp-like.)

There is a good bit of truth to the conventional wisdom—indicas do tend to produce stuporous, narcotized highs, while sativas tend to produce giddy, exhilarating ones—but there’s a bit more to it than that. And those relying on the good old sativa = up / indica = down as a guide may find themselves in for a surprise.

The differences between the two species are evident to the naked eye. Sativas typically are tall and lanky, with long narrow leaves, while indicas are shorter, stouter, bushier, and have thicker, stubbier leaves. That, says Krymon deCesare, chief research director at Steep Hill Halent Lab in Oakland, is because the two cannabis species developed in different environments.

He told High Times that marijuana’s origins are in South and Central Asia, and that the plant differentiated itself into distinct species to accommodate different humidity regimes. The thin, lanky stems and long leaves of sativa plants allow the plant to respirate more efficiently and prosper in high humidity, while short, squat indicas evolved to deal with hot, dry conditions.

Thus, landrace varieties — “pure” original strains of indica, such as Afghani and Hindu Kush—  developed in the dry foothills of the Himalayas, while pure sativas evolved in humid lowlands and river valleys. But in the US market today, landrace strains are a rarity. The vast majority of the weed grown and consumed in the US is one indica-sativa hybrid or another.

That makes sense, for both growers and consumers. For growers, even if they want that trademark stimulating sativa high, they don’t want to spend extra weeks waiting for it to mature, so they use hybrid strains with varying amounts of indica that will ripen faster than a pure sativa.

For consumers, hybrids are similarly attractive. You can get couch lock and the giggles without being thoroughly sedated or completely zonked. Pot buyers will look for a hybrid they think will satisfy their desires: Do they want to be mainly up with just a hint of a body high? Then they may want a sativa-heavy hybrid such as Haze, Blue Dream or Strawberry Cough. Do they want to get stuporous, but maybe laugh a little, too? Then they’ll go for indica-dominant strains such as Hash Plant, Blueberry or Girl Scout Cookie.

But you don’t always get the high you think you’re going to get. That’s because growing conditions make a difference, and even stabilized strains, pure or hybrid, can exhibit new traits when grown in conditions to which they are not accustomed. But part of the reason is a bit stranger—and calls into question the traditional reliance on the indica/sativa distinction.

“The terms sativa and indica are only really valid for describing the physical characteristics of the cannabis strain in a given environment,” deCesare told High Times. “They are not nearly as reliable as terms for making assumptions about energy versus couch lock.”

He explained that the effects of THC, whether in indicas or sativas, are the same: It creates a euphoric, uplifting sensation when smoked. That sound pretty much like a sativa high, so if indica contains THC just like sativa, why does some indica leave you in a prostrate stupor?

DeCesare has the answer. While both indicas and sativas generally contain the full complement of cannabinoids in addition to THC, and the same terpenes—chemical compounds that create odors and essential oils—some indicas are especially heavy in one terpene that flips the switch on the up THC high. That terpene is myrcene.

“We found consistently elevated levels of the terpenoid myrcene in C. indica, as compared to C. sativa,” he explained. “Myrcene is the major ingredient responsible for ‘flipping’ the normal energetic effect of THC into a couch lock effect.”

In fact, deCesare says, myrcene rather than THC is probably the most important variable in creating the psychoactive differences between indica and sativa. That conclusion is based on the analysis of more than 100,000 marijuana samples taken over the past seven years.

He points to the theory of the “entourage effect,” developed by his colleague, Ethan Russo, as a better explanation for different highs than the indica/sativa distinction. That theory postulates that it is the combination of different cannabinoids and terpenes working together that creates the distinctive highs of various strains.

Myrcene isn’t limited to marijuana. It exists in many fruits and plants around the world, and some of them provide additional support to deCesare’s theory.

“Notice the warm, relaxed feeling you get from a couple of hoppy beers?” deCesare asked. “That effect is, to a good extent, due to the myrcene present from the hops.”

In his work at Steep Hill Halent, deCesare has found that myrcene levels make a difference. A level below 0.4% doesn’t seem to effect the “upness” of the high.

But go beyond that and “the strain becomes increasingly more sedative and stony,” deCesare noted. “OG Kush is considered by most to be a strong couch lock flower at about 1.25% myrcene. A few strains have a myrcene content in excess of 3%. Other chemicals may well play minor roles in the couc hlock effect, including CBD, CBN and linalool, when they are present in couch lock strains—but they aren’t always present or as influential.”

Once marijuana is widely legal, myrcene content should be part of the labeling, he suggested.

“Moving forward to a time when the USDA and FDA oversee cannabis-distribution regulations,” he replied, “they will insist on accurate labeling to assure that if a customer purchases an energetic strain—or a couch lock strain—then what they get is what they paid for. And the only reliable way to make this determination is by lab-testing for myrcene content.”

Can’t wait for that USDA-certified couch lock sticker.