인터넷으로 검색해 보니 근로소득(earned Income)만 포함 된다고 합니다. 따라서 불로소득은 제외되고, 보너스는 포함될 것 같습니다.
The Social Security tax only applies to your earned income, such as wages, bonuses and self-employment income. All of your unearned income, like capital gains, interest and dividends, are exempt from the Social Security tax, regardless of how much income you have.