Tashfeen Malik, left, and Syed Farook at O’Hare International Airport in Chicago on July 27, 2014. (U.S Customs and Border Protection)
Audrey Alexander is a research fellow at the GW Program on Extremism. She specializes in the radicalization of women and helped write "ISIS In America: From Retweets to Raqqa."

When news broke that Tashfeen Malik and Syed Farook had carried out the shootings in San Bernardino, Calif., people were shocked. Mass shooting are very rarely committed by more than one person in the United States.

But really, we shouldn’t be surprised. Marriage is a perfect hotbed for radicalization, and couples often push each other to behave more radically than either would on their own. While men are often portrayed as instigators, radicalizing their supposed lovers, history suggests that these interpersonal dynamics are rarely that simple.

That’s because radical relationships create a mutually reinforced echo chamber where dissent and concerns are easily silenced by the consolation of trusted companions. While romantic duos are not the most common form of terrorism, this trend deserves recognition. Tight-knit groups of friends are known to be influential regarding radicalization, and few relationships are closer than marriage.

See, for example, Amanda and Jerad Miller, a married couple of reportedly anti-government white supremacists. In June 2014, they killed two police officers at a pizzeria before shooting a civilian in a Las Vegas Walmart. In 2005, Belgian Muriel Degauque and her husband, Issam Goris, tried to carry out suicide bombings in Iraq. While Goris’s attack was thwarted, Degauque killed herself and injured an American soldier.

More recently, Hayat Boumeddiene, the wife of the shooter in the January 2015 attack on a Kosher supermarket in Paris, allegedly fled to Turkey and crossed the border into Syria days before the January 2015 Paris attacks. French authorities say that Boumeddiene exchanged more than 500 phone calls with the wife of Cherif Kouachi, one of the Charlie Hebdo gunmen.

These partnerships are dangerous. Rather than using communication tools such as online platforms and messaging systems, tight-knit partners are more likely to interact in person while plotting attacks, which may easily circumvent detection by intelligence agencies.

Not only does this phenomenon occur in romantic relationships — and with parents — but also within families.

Within the foreign fighter movement in support of the Islamic State, entire families have migrated to Islamic State-held territory. Australian couple Khaled Sharrouf and Tara Nettleton, for example, relocated to Syria with all five of their children in December 2013. Mississippi newlyweds Jaelyn Delshaun Young and Muhammad Oda Dakhlalla used the guise of a honeymoon to mask their attempt to join the “caliphate.” The newlywed couple communicated with an FBI informant who posed as a fellow Islamic State supporter and was arrested on their way to Syria.

Perhaps the most notable sibling pair, the Tsarnaev brothers, carried out the 2013 Boston Marathon bombing. The now-deceased older brother, Tamerlan, is often credited for the duo’s ideological transformation while the younger brother, Dzhokhar, who is facing the death penalty, is responsible for his participation in plotting and executing the attacks.

Extremism is not limited to individuals and lone actors, nor does it only exist within diffuse networks. Romantic partners and family members are extremely influential with regard to radicalization. Although certain individuals may be dominant agitators, radicalization is rarely a one-way street.