In Yaa Gyasi’s new novel, Transcendent Kingdom , a Stanford Ph.D. candidate named Gifty studies reward-seeking behavior in mice and the mysterious synapses that can lead to addiction or depression. She does it because her brother, Nana, was a gifted basketball player before an injury led to an OxyContin addiction and eventually to a deadly heroin overdose. And she does it because her mother, a Ghanaian immigrant, is depressed and living in her bed. Gifty wants to better understand the science of