Nonetheless the study invented two new categories of person: the “young casual marijuana user” and the young non-marijuana user. This is the latest example of turning to brain imaging to make something seem objective. Establishing brain differences among certain groups highlights the uniquely ignoble political history surrounding the criminalization of a plant.

Marijuana has a particularly frustrating existence in the U.S. There are more people in federal prisons for marijuana offenses than for violent offenses. According to the ACLU, nearly half of drug arrests in 2012 were for marijuana—close to 750,000. And almost half of those arrests were for possession alone. Almost $4 billion is spent annually on the arrest, prosecution, and incarceration of marijuana offenders. And these statistics are egregiously skewed according to race. Police in the biggest American cities like Los Angeles, Chicago, and New York arrest blacks for marijuana possession at a rate seven times greater than their arrest rate for whites, despite that marijuana use rates do not differ between blacks and whites.

Given these injustices, the kind of science that the Harvard study exemplifies offers a cautionary lesson in how neuroscience cannot be an unbiased form of knowledge that, as some posit, “speaks for itself.”

From Josiah Clark Nott and George Robert Gliddon's

Indigenous races of the earth (Wikimedia)

We first started seeing the use of seemingly “objective” metrics to make ultimately social claims about racial inferiority in the 19th century. One clear example of this was the physical anthropology of the mid-1800s, which, among other things, sought to establish race-based theories of “intelligence” and “moral reasoning” based on physical attributes like the shapes of skulls.

Within this tradition, the influential American scientist Samuel George Morton developed “craniometry” in the 1830s to prove that humans were actually made up of separate species, hierarchically ordered in terms of intellectual capacity as determined by skull volumes. Morton’s theories were recruited to justify slavery in the face of growing moral repugnance against it. After his death in 1851, the Charleston Medical Journal eulogized him accordingly: “We can only say that we of the South should consider him as our benefactor, for aiding most materially in giving to the negro his true position as an inferior race.”

It is clear to us now that racist ideology was in play before anyone in the 19th century grabbed calipers and began measuring people’s skulls in the name of scientific inquiry. Yet we do not exactly let 19th-century racist anthropology off the hook because its practitioners adhered to the scientific standards of the time—so why should we treat today’s sciences differently? To what extent is the neuroscience we do today just a technologically-savvy version of a craniometry or phrenology that still reinforces social categories? As the late paleontologist Steven Jay Gould put it, “Shall we believe that science is different today simply because we share the cultural context of most practicing scientists and mistake its influence for objective truth?”