Psychologist Robert Sternberg never forgot the low IQ score he earned as a child. Now his theory of “successful intelligence,” which he says is a better index of brain power, will be put to a real-life test. This fall undergraduate applicants to Tufts University, where Sternberg is the college dean, will be given a chance to write an optional essay and attend an in-person session where they will respond to videos and pictures, leading to an index for each volunteer.