From what I’ve read, William Grant Still – a fine composer who, by the way, studied at Oberlin College, just up the road from here – implied pretty strongly that Gershwin had gotten the main melodic idea for “I Got Rhythm” from Still. Gershwin may have heard it as part of Still’s improvisation in the pit during “Shuffle Along.”

“I Got Rhythm” was a huge hit when Gershwin used it in “Girl Crazy” in 1930. Still seems to have been quite conscious of the effect it would have on his listeners when he incorporated Gershwin’s snappy, syncopated version of the motif into the scherzo of his Afro-American Symphony later that same year – effectively borrowing it back!

Whether Gershwin used any of Still’s material for “Porgy and Bess,” I can’t say, not having studied both side-by-side. However, it does appear that Gershwin (and some of his colleagues, both in the US and in other nations) felt pretty free to treat everything they heard that way as public domain or folk music.

Not being a legal expert, I’m not going to try to pass judgement on whether any of that rises to plagiarism or copyright violation. I will say though that conscious and unconscious quotation and borrowing has been going on among musicians and composers for centuries, and I think you could argue pretty persuasively that such cross-pollination has made music richer.

In earlier times, before copyright laws, the “borrowees” usually considered this a compliment. (Indeed, they had little choice.) Today they call it copyright infringement and bring in the lawyers. Determining which is the better system will be left as an exercise for the reader.

To whatever extent Gershwin did incorporate musical ideas from William Grant Still and others, I think it made him a better composer. Some may say that doesn’t excuse such behavior, but it certainly enriches our musical legacy from Gershwin.