Killer rhymes: Can rap lyrics be used as evidence in murder trials?

Tom Jones sang about stabbing a cheating girlfriend named "Delilah" and an abusive boyfriend was poisoned in the Dixie Chicks' "Goodbye Earl." It's not abnormal for musicians to sing about committing murder.

But for some rappers, taking part in that tradition has landed them behind bars. Since 2012, researchers have identified at least 31 cases in 16 states where rap lyrics were introduced as evidence.

In New Jersey, Vonte Skinner was convicted of attempted murder during a drug-related shooting in 2005. During the trial, prosecutors read out loud 13 pages of rap lyrics -- some written years earlier -- from Skinner's notebook.

Ed Barocas of the ACLU told CBS News' Don Dahler that the lyrics prejudiced jurors. In a brief filed with the state Supreme Court, his group argued that rap music -- even when vulgar and violent -- is free speech and should be inadmissible.

"What we have here are lyrics written very generally, not about the individual who is the victim of a crime," he said. "No matter what you think of it, on that level, it's something that is protected under our Constitution."

CBS News' legal analyst Jack Ford joined "CBS This Morning" to discuss whether the lyrics are just music or evidence of a crime. He told the co-hosts that it's not really a First Amendment fight, even though it sounds like it should be.

"It's really a fight about whether the lyrics are actually factually relevant in to the crime," he said. "We all have First Amendment rights, but that doesn't mean they're not without consequences. You know, the Supreme Court said famously 'You don't have the right to falsely scream "fire" in a crowded theater' because you could harm people as a result of that."

He used Johnny Cash's "Folsum Prison Blues" as an example. He said it's not applicable because Cash wasn't accused of killing a man in Reno, but if had been and then sang about the murder afterwards, then the lyrics might have been admissible as evidence against him.

"The court has to start off saying 'Are these lyrics here we're talking about directly related to the facts in a crime.' If they are, then maybe they might be admissible," said Ford.

He said the problem with Skinner's case is that the lyrics did not match the crime.

"They (the lyrics) were talking about things that happened before the crime happened. Factually things that were very different," he said.

Ford explained that a defense attorney can argue that using the lyrics is just an attempt to show that the accused is a bad guy or to "create in the minds of jurors this idea that gangster rappers are disciples of violence" and because of that it's more likely that they committed a crime.

"The law doesn't allow us, when we're trying someone, to say 'Let's put in the game all the bad things this person has done in their life. Show the jury he's a bad guy and therefore he probably did this,'" he said. "It has to be directly related to the facts of the crime.

To see Jack Ford's full interview, watch the video in the player above