When you think of American music in the 1990s, many people would think of Seattle, guitars, and hair.

Grunge grew out of the suburban garages of the Pacific north-west and went on to transform mainstream music, via bands like Pearl Jam and Nirvana.

But that was the early 90s — and on the alternative charts. The story of pop music in the 90s, the stuff that was dominating the airwaves and television screens, is actually quite different.

It's a story about hip hop.

"There is a really obvious shift towards hip hop, in a really big way, as the 90s gets underway," Dave Carter, a senior lecturer in music technology at the University of Tasmania, said.

"Things like grunge are really under-represented, even though lots of people would associate grunge and Nirvana with that decade."

You might think hip hop, the most popular genre in music today, has always been a mainstream concern. The most recent Hottest 100 winner is Kendrick Lamar, after all.

But it was about 1993, following what's considered the golden age of hip — led by groups such as Beastie Boys, N.W.A and Run-DMC around the end of the 1980s — that the genre graduated to the big leagues.

Insane In The Brain, by Cypress Hill, became a crossover hit that year, as did records by Salt-N-Pepa and Dr. Dre.

"Then Snoop Dogg is charting in '94, Tupac is charting in '94, R. Kelly is charting," Dr Carter said.

"By '97, four of the top 10 are hip hop artists, or hip hop artist collaborations, and the charts are just a lot more full of that kind of music."

Hip hop was laid back — and then so were the charts

Dr Carter suspects MTV's embrace of the genre might have had something to do with this trend. It had a young, and — most importantly — national audience, breaking down barriers in a genre defined by regional specificity.

He also suggested there might be a deeper reason: that hip hop, known for its grand displays of wealth and fame, was unafraid of capitalism.

But Hau Latukefu, host of triple j's Hip Hop Show, said there had long been a DIY ethos in hip hop, pioneered by groups like Public Enemy, that was unwilling to capitulate to the mainstream.

"'We're going to do it our way' — that was the attitude, much like punk, for a long time," Latukefu said.

Eventually, things changed, but there was resistance.

"It took a while for the culture to find a good balance between pushing your art, or pushing a certain agenda, with making money off of it."

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As hip hop got bigger, it started influencing popular music in general.

Hip hop tends to be slower; it's hard to rap over music that is as fast as dance or punk.

And so, in the 90s, pop music itself started to slow down.

Average tempo of songs in the US Hot 100 each year (in beats per minute). ( Double J/Columbia Business School/INSEAD )

In a recent study, Columbia Business School's Michael Mauskapf and INSEAD's Noah Askin analysed 27,000 songs that appeared in the US Hot 100 chart between 1958 and 2016.

What they found was that pop music reached its all-time-slowest point in 1996.

"Just by the nature of [hip hop] being much, much slower than the 120 [beats per minute] in the preceding decade, that average is going to get pulled down," Dr Carter said.

That year, you also had Babyface-style R&B slow jams like Mariah Carey and Boyz II Men's One Sweet Day, and that probably brought the average down, too.

Did weed have something to do with that slowdown?

Drug references in hip hop are cyclical, and in the early-to-mid 90s it was all about marijuana.

"Spurred by the arrival of Cypress Hill's self-titled debut album in 1991 and Dr. Dre's The Chronic in 1992, marijuana's popularity increased exponentially, peaking in 1993 as the most popular drug in hip hop," the lyric website Genius, analysing its own database, reported in 2016.

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One study found that, after 1988, drug references became more frequent in the lyrics of pop songs — "an increase that mirrors the explosive advance of hip hop into music mainstream during the 1990s," the authors wrote.

Another study of rap songs in the year 1996-97 found the most commonly referenced drug was marijuana.

"It definitely influenced the sound and tempo of the music," said Latukefu, pointing to the down-tempo, "smoked-out" sound of Cypress Hill.

There is precedent for this kind of effect in hip hop.

In Houston, in the early 90s, there was chopped and screwed, a very sedate sub-genre Latukefu said was a direct result of the use of "purple drank", or cough syrup, which apparently slowed down the brain.

Dr Carter said it was possible weed did play a part in the slowing down of hip hop — and therefore pop more broadly — given a similar recognised link between faster tempos in punk and amphetamine use in that genre.