Popular opinion asserts that tetrahydrocannabinol (THC) levels in cannabis have increased over the decades. We remember flower from the 1970s as gentle and mellow, declaring that today’s products stagger by comparison. Some may blame breeders for maximizing this chemical at the expense of others. Stuyt  declares that “The primary problem with the current available cannabis in dispensaries in Colorado is that the THC content is not like it used to be. Prior to the 1990s it was less than 2%.”
A recent review  sought to determine if and by how much concentrations of THC and cannabidiol (CBD) have shifted. The researchers thus analyzed different reports on cannabis flower and resin from 1970 to 2017. The studies they reviewed generally used gas chromatography for analysis, which converts tetrahydrocannabinolic acid (THCA) and cannabidiolic acid (CBDA) into their neutral forms. That said, the research authors don’t mention this, and not all studies surveyed specify the analytical testing technique used. They do mention that “The majority of articles reported data on samples of seized illicit material…” The Drug Enforcement Agency (DEA) prescribes gas chromatography for seized cannabis.
Initially, they filtered through almost 4,000 articles with apparent relevancy and identified about 120 worth full-text examination. Of these, 12 papers met their criteria for THC, and 5 also measured CBD. The criteria included the reliability of sampling and analytical protocol; studies had to measure THC and CBD concentrations at three time points to qualify. The researchers broke herbal cannabis into “traditional herbal” or “sinsemilla.”
The studies spanned multiple countries, namely: USA (5 studies), UK (1 study), Netherlands (2 studies), France (1 study), Denmark (1 study), Italy (1 study), and New Zealand (1 study). Herbal cannabis (including “sensimilla”) samples numbered 66,747, and resin samples numbered 17,371.
THC levels in cannabis flower increased 0.29% per year, or 2.9 mg per gram, from 1970 to 2017. In cannabis resin, the increase was higher at 0.57% per year, or 5.7 mg per gram, from 1975 to 2017. No significant changes were found in CBD content although the date range was more limited (i.e., 1992 to 2017). On the surface, particularly looking at resin, this seems to support the idea that breeders have elevated THCA (tetrahydrocannabinolic acid, the dominant cannabinoid in flower) in cannabis over the years.
Nonetheless, within the categories of “high-THC sinsemilla,” and “low-THC traditional,” significant increases in THC were not observed. Increases were observed, however when comparing traditional versus sinsemilla cannabis. These high-THC varieties came to dominate market share, thus elevating the data. Therein lies the crux of the study: by differentiating between low-THC “traditional” cannabis and high-THC “sinsemilla,” the researchers found that, over the years, market preference for high-THC cannabis increased THC rather than breeding more THC into plants. Otherwise, the levels of THC in “sinsemilla” would also have increased significantly over the timeframe studied. The study challenges the assumption behind “Prior to the 1990s [THC] was less than 2%.” That these plants from the past contained massive amounts of CBD is also less than evident.
Overall, the researchers conclude, “Increases in THC were greatest in cannabis resin whilst increases in herbal cannabis are primarily attributable to a greater market share of high‐THC varieties…” 
1- Stuyt E. The problem with the current high potency THC marijuana from the perspective of an addiction psychiatrist. Mo Med. 2018;115(6):482-486. [Impact Factor: 6.34; Times Cited: 10 (Semantic Scholar)]
2- Freeman TP, Craft S, Wilson J, et al. Changes in delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) concentrations in cannabis over time: systematic review and meta-analysis. Addiction. 2021;116(5):1000-1010. doi:10.1111/add.15253. [Impact Factor: n/a; Times Cited: 2 (Semantic Scholar)]