Jacinda Ardern was 37 when she was elected prime minister of New Zealand, becoming the world's youngest female head of government. After leaving office two years ago, she moved to Boston, where she's serving as a fellow at Harvard University. She's also written a new book, "A Different Kind of Power." Ardern talks with "Sunday Morning" national correspondent Robert Costa about her experience leading a nation (including passing a ban on semi-automatic firearms); the importance of exhibiting kindness and empathy in politics; and the challenges of being a working mother, after giving birth while in office.