Democratic presidential candidate Cory Booker’s tax returns show most of his wealth stems from lucrative speaking engagements and royalties.

Booker, the 2020 candidate who has yet to make a splash in the crowded Democratic field, released 10 years of tax returns on Wednesday after numerous other candidates released their records in recent weeks.

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The New Jersey senator reported income of $152,715 in 2018 for his salary, $22,781 in taxes which amounts to an effective tax rate of 15 percent, significantly lower than Sen. Kamala Harris’ 37 percent or Sen. Bernie Sanders’ 26 percent.

Most of Booker’s wealth comes public speaking fees and royalties, including $2 million in speaking fees between 2009 and 2014, nearly $1 million in royalties from 2015 to 2017 after the release of his book, “United.”

But the lower effective tax rate may have something to with Booker’s sizeable charitable donations. In 2018, he donated $24,000 to charity. In total over the 10-year period, the senator donated nearly $460,000 to various organizations and causes.

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This appears to be significantly more in proportion than his opponents like Sanders or former Congressman Beto O’Rourke, who faced questions over his household giving to charity just $1,166 in 2017, or about 0.3 percent of their income that year.

At the same time, more than half of those donations made by Booker came in 2013 amid criticism of his role in the founding of a social media company called Waywire, prompting him to give massive amounts of stock to charities in his city.

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The New York Times revealed that Booker’s wealth at the time – $5 million – consisted mostly of shares in the company.

The Associated Press contributed to this report.