> I'm not sure why employers must prove the relevance of IQ tests but not work sample tests
If challenged in court, and if the work sample test is shown to have a disparate impact such that its use would be illegal discrimination if it were not tailored to the job, they do have to show this for work sample tests. Your premise is simply false.
(OTOH, the disparate impact of IQ tests is more readily established by a plaintiff since there are numerous, readily available studies. So the work of establishing the threshold issue which requires the employer to prove relevance has largely been done by the plaintiff for IQ tests.)
> given that the former are better predictors of job success
The legal standard addresses the specific job for which hiring is being done, not a generic job. Interestingly, your source indicates that the types of jobs where IQ test challenges have notably occurred are those where the test is least relevant per the source you cite (unskilled/semiskilled jobs).
Let's imagine that tomorrow I come up with a magical super-reliable method of interviewing, which works better for most jobs than existing specific methods. Then I'm fucked! My method will certainly have disparate impact (because it has nonzero correlation with IQ), be not very tailored to any particular job (because it's magical), and there will be numerous available studies (making it easier to sue me as you explain). So tell me, what incentive do I have to invent such a method?
> Let's imagine that tomorrow I come up with a magical super-reliable method of interviewing, which works better for most jobs than existing specific methods.
If you can demonstrate this is the case for the specific jobs where you want to apply it, you will then have no problem, even if it has a disproportionate negative impact on a protected class.
Only in the same sense that an unjustly accused person will "have no problem" as long as they're actually innocent. In practice, they do have a problem. Firms are afraid of using IQ tests and being the first against the wall.
If challenged in court, and if the work sample test is shown to have a disparate impact such that its use would be illegal discrimination if it were not tailored to the job, they do have to show this for work sample tests. Your premise is simply false.
(OTOH, the disparate impact of IQ tests is more readily established by a plaintiff since there are numerous, readily available studies. So the work of establishing the threshold issue which requires the employer to prove relevance has largely been done by the plaintiff for IQ tests.)
> given that the former are better predictors of job success
The legal standard addresses the specific job for which hiring is being done, not a generic job. Interestingly, your source indicates that the types of jobs where IQ test challenges have notably occurred are those where the test is least relevant per the source you cite (unskilled/semiskilled jobs).