Two of the suicide bombers in the Brussels attacks were brothers, which leads Discovery to ask: Why is it relatively common for sets of brothers to become terrorists? The Tsarnaev brothers in the Boston Marathon bombing and the Kouachi brothers in the Charlie Hebdo attack are two recent examples, but there are more: Slate and the Guardian have both reported previously on this phenomenon, pointing out that six of the 9/11 attackers were brothers and that there are "many" other sets of brother terrorists, including two of the 2015 Paris attackers. A decade ago, US military intelligence said that having a close family member involved in violent militancy was the biggest predictor of whether a person would get involved. The reasons likely have to do with multiple factors, including genetics and environment. "Neurobiological, personality, social, and cultural" factors all contribute to terrorism, one neuropsychologist explains to Discovery.
There are two genes that increase a person's risk of being violent, recent research found, and it's possible those genes are inherited in families. Upbringing may also play a role—a study finds that children raised in unstable or abusive families are often desensitized to violence, not emotionally affected, for example, by witnessing violent acts like beheadings; obviously, brothers would have been raised in similar home environments. "One brother is usually a stronger character and the leader of the two," the neuropsychologist explains. "The other brother is a follower and is influenced by the leader-brother. The follower wants to please the leader and obtain the respect of the leader." Counterterrorism experts point out that whether they're brothers or just friends, terrorists tend to form tight-knit groups, helping to radicalize each other as they conspire. In the case of the Brussels bombings, a set of brothers from Belarus were also named as attackers in the Russian media, Ukraine Today reports, but they say that was simply a false rumor. Belgian officials do not consider them suspects. (Read more Brussels attack stories.)