Researchers at North Carolina State University have defined the most popular themes in music based on the decade in which they were released. They tool the Billboard "Hot 100" for each year, inputted the lyrics from all the songs and had a computer analyze the most common themes from each decade. Kind of interesting:

1960's: Pain, Nostalgia & Rebellion (it was a turbulent time)

1970's: Nostalgia, Rebellion & Jadedness

1980's: Loss, Aspiration & Confusion

1990's: Loss, Inspiration & Escapism 

2000's: Inspiration, Pain & Desperation (post-9/11...kind of makes sense)


Here's a link to the entire article: