Under New York state law, a confession can be enough to convict someone as long as authorities can establish that a crime occurred.
False confessions are a longstanding legal phenomenon. Examples range from the more than 200 people who came forward to claim to kidnapping the infant son of aviator Charles Lindbergh in the 1930s to the 2006 episode in which a man falsely said he'd killed JonBenet Ramsey, the 6-year-old beauty queen found dead in her parents' Colorado home a decade earlier.
Fishbein said he planned to have expert witnesses explain why people sometimes admit to crimes they didn't commit.
"It's a hard concept to understand. But it's a reality. And it's a scary reality," Fishbein said.
Asked whether Hernandez still believes his confession, the attorney answered, "He's pleading not guilty."
From the outset, Hernandez's his wife, Rosemary, told investigators that he had a history of hallucinations, said an attorney representing the family, Robert Gottlieb.
The wife and the couple's daughter, Becky, "have lived with him and seen firsthand that type of behavior," Gottlieb told reporters outside court.
"They support him 1,000 percent," the lawyer added. "They don't believe the so-called confession."
Etan's parents had a court declare the boy legally dead more than a decade ago, allowing them to sue convicted child molester Jose Ramos in the boy's death.
Ramos was found responsible — a ruling made because he didn't entirely cooperate with questioning during the lawsuit — but it's unclear how that finding could now factor into the prosecution of Hernandez.
Ramos, now 69, had been dating the boy's baby sitter in 1979 and was considered a suspect. He was later convicted of molesting two different children and was in prison in Pennsylvania prison for more than a quarter-century.
Ramos had been scheduled to be released last week, but he was immediately rearrested on a charge of failing to register properly as a sex offender; authorities said he'd lied about where he planned to live, giving an address a relative vacated long ago. He was ordered to stand trial Thursday for allegedly providing authorities with a bogus address.
Etan's father, Stanley Patz, said Wednesday that the family wasn't commenting on the case.