Are there tell-tale signals to a shooting spree such as the one in Colorado last week? Psychologists say it's nearly impossible to accurately predict human behavior, although they do agree that parents, teachers and friends need to heed warning signs that can signal the possibility of extreme violence. "It's awfully hard to generalize about these things," said Laurence Steinberg, professor of psychology at Temple. "Some have long histories of depression, others don't. Right now, we don't know yet about (Holmes) history."