That's a separate document, which is not IRS form 1040. The Form 1040 Instructions document is informative, not normative; you can compare for instance the qua

That's a separate document, which is not IRS form 1040. The Form 1040 Instructions document is informative, not normative; you can compare for instance the qualifying child tests here: https://www.irs.gov/publications/p501#en_US_2022_publink1000220886 https://www.irs.gov/publications/p501#en_US_2022_publink1000... which as you see do not include gender. To be specific you may refer to 26 USC §152(2), which only specifies gender-specific terminology in (2)(b), where they mandate "brother, sister, stepbrother, or stepsister of the taxpayer". I'll be writing my representatives asking this language be changed to "sibling or stepsibling of the taxpayer." In the meantime, Form 1040 itself does not and has never mandated a description of "son" or "daughter."