There’s a reason interviewers in the US won’t (or shouldn’t) even ask if the candidate has a spouse. If they ask something about that specifically, and the answer indicates some kind of protected status (a man says “my husband” or reveal which place of worship they got married in) and they then decline the candidate, the candidate could make the claim they were denied because they’re gay or practice whatever religion or something else.

Asking personal questions could be seen as a way to elicit information about a protected status and thus give a rejected candidate ammunition for a claim, whether warranted or not.

It’s best to just keep questions focused on the workplace.

Never ever prompt someone to discuss personal relationships in an interview. The moment the conversation drifts into religion, family status, child count, sexuality or gender makeup, or any number of other things, you can easily run afoul of state or federal laws, or open yourself to discrimination lawsuits.

Discrimination of sexual orientation, for example, depending on how it's asked. Just one of those areas best left alone in an interview

Employer fishing to see if the person is married, which will require additional dependents on health insurance. Married is possibly more likely to have kids and take more time off for them or maternity/paternity leave.

Women in a committed relationship can enter a medical situation that renders then unable to work for 6-9 months, + 2 - 3 years of leave afterwards. Men don't, that's just a month or two twice.

It is illegal, and in my book also immoral to deny such a candidate, but the other side of the coin is there.

Working in selection, I can say it’s more nuanced than that. Any measurement can be used as long as it is relevant to the business and related to performance. For example, you’re fine to reject people based on height if you’re hiring basketball players and being higher predicts scoring more points. Or even reject people based on gender (or other protected classes) if you can demonstrate that that specific group is absolutely necessary for you e.g. you want a counselor working with sexual trauma survivors and have evidence that matching patients to counselor on gender gives meaningfully better results for said patients.

The specific cases you mention and the finer point is how do you demonstrate the necessity of a measure? Is high general IQ absolutely necessary for SWEs? Or is it enough to have a high logical reasoning, but don’t need spatial? Do you really need high IQ or is it enough to have a lot of practical experience with hands on skills? Do you need higher IQ to do zero to one development vs code maintenance? The devil’s always in the details with these kinds of questions, and it’s definitely not a blanket “you can’t use anything”.

IQ testing in white collar US employment is not unlawful and several household name companies openly perform general cognitive assessments; the companies that provide those tests have logo crawls just like every other product company. Griggs doesn't say what you think it says.

IQ testing is uncommon in US employment because it doesn't do a good job of selecting candidates, not because it's unlawful.