This chapter argues that throughout history many religions have proved themselves capable of sparking and fueling hostility toward outsiders and even toward people in the same faith who are viewed as unacceptable for one reason or another. We examine recent manifestations of extremism in Christianity, Hinduism, Buddhism, Judaism, and Islam, explaining that analysts often disagree about the extent to which religious beliefs and institutions are causally important. Key terms such as religious extremism, fundamentalism, radicalization, and terrorism are defined, noting the crucial importance of maintaining a distinction between the religious extremist and the extremely religious. Though we suggest researchers face many methodological challenges, we explore a broad range of empirical studies on related topics. The chapter also reviews theory and research on why and how people become religious extremists. We further draw on the psychology of radicalization, arguing that nowadays most scholars believe that there are cognitive and behavioral processes at work. Some people may move directly to carrying out terrorist deeds without acquiring much group ideology or religious belief.

Here, I use the phrase ‘intelligent universe’ to refer to all intelligent entities everywhere. Whether the set of all such entities overlaps with, or is a subset of, the biological universe depends on whether we include artificial intelligence in it. I focus here on biological intelligence. On Earth, evolution to high intelligence has proceeded via a series of milestones. These include: multicellularity, bilaterality, brain, and dexterity. To what extent does evolution towards high intelligence elsewhere proceed via the same milestones? I suggest that similar steps would often be found to characterize evolution on other inhabited planets, providing it can continue for long enough. I put forward the hypothesis that there are at least a trillion radio-level intelligences in the observable universe right now. Then I consider the possible implications of ‘first contact’ between humans and one of them. Such contact could pose a threat for human survival. Finally, I look at home-grown threats, including the fixed mind-sets that underlie religious fundamentalism and science denial. I end by urging a robust defence of both science and humanity against such unthinking views.