We rely on 100 years of data!
The filings with the IRS disclose the name of the individual. We compare the name and the middle name with a database from the US Social Security Administration - SSA, which contains more than 95,000 different names, from 400 million entries, classified by gender, since 1937. We only assign a gender when the probability is 95% or above.
Some names have been reported by both sexes for the past 100 years. Examples are: Rene (69% male / 31% female), Taylor (74% female / 26% male), Elliot (92% male / 8% female). You can use this free tool from Giving Tech Labs to check some names.