One of the big questions still surrounding the Boston marathon bombings, which killed three on April 15, is how involved younger brother Dzhokhar Tsarnaev was in the planning.
Some think his older brother Tamerlan, who died during the police chase days after the bombing, was the brains behind the operation.
Psychologist and language analyst James Pennebaker, of the University of Texas at Austin, did an analysis of Tsarnaev's tweets and presented the data at the 2013 American Psychological Association conference in Honolulu on July 31.
Note: These studies are preliminary and not peer reviewed.
Tsarnaev started tweeting in October of 2011, and by February of 2012 he was posting about 50 times a month. He posted a total of 867 tweets before being arrested, 11 of them after the Boston bombing.
Pennebaker analyzed the language and words used in these tweets to see if Tsarnaev knew about the bombing, and when he knew. They were looking specifically for I-words, like I, I've, I'm.
Here's the graph from Pennebaker's presentation:
You can see there's a steady decline in the use of I-words over time, especially in October of 2012.
"Consistent with previous work on precursors to aggressive acts, Dzhokhar's tweets showed a large drop in I-words as April approached," Pennebaker told Business Insider in an email. "The pattern suggests that he made a personal conviction to carry out violent actions as early as October, 2012."
Tsarnaev has been indicted on 30 counts related to the bombing, and pled not guilty.
SEE ALSO: BOSTON MASSACRE: The Full Story Of How Two Deranged Young Men Terrorized An American City
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