Given a highly complex job profile, could an employer legally filter job candidates based on their IQ or the results of similar intelligence tests?
In short: Yes, it's legal to use intelligence-based testing in a hiring decision, provided that certain requirements are met.
As Criteria Corp, a pre-employment testing company, summarises:
Like all the other elements of a company's hiring process, pre-employment testing is subject to a series of federal laws governing hiring practices. The most important legal standards related to testing are contained in the Uniform Guidelines on Employee Selection Procedures (UGESP), which explicitly recognizes the right of employers to use pre-employment tests to make hiring decisions as long as those tests are job-related. The UGESP provides interpretive guidelines for federal agencies charged with enforcing the Civil Rights Act, and is designed to ensure equitability and prevent unfair discrimination in hiring. These guidelines inform the decisions made by the Equal Employment Opportunity Commission (EEOC), the federal agency that enforces federal employment discrimination laws.
The UGESP mentioned here is a codified set of standards and guidelines based on the Civil Rights Act of 1964. It's been designed for and adopted by federal agencies and is therefore also a best practice for the private sector. One of the most important clauses in the mentioned Act is the following:
nor shall it be an unlawful employment practice for an employer to give and to act upon the results of any professionally developed ability test provided that such test, its administration or action upon the results is not designed, intended or used to discriminate because of race, color, religion, sex or national origin
Source: Title VII of the Civil Rights Act of 1964, SEC. 2000e-2. [Section 703] (Unlawful Employment Practices), (h)
There may be other relevant clauses in the full text but a full analysis would take us off-track. The landmark legal case though is Griggs v. Duke Power Co. from 1971. In brief, the result there was that employment requirements (and the tests checking for them) should pertain to applicants' ability to perform the duties that would be required of them.
To use a crude example: higher-order thinking tests and similar intelligence tests can't be used when hiring manual labourers because it would disproportionally affect poorly educated demographics, effectively discriminating without a justified reason. Such a company could set up a "professionally designed" test that asks about warehouse safety and OSHA best practices and the like for supervisor jobs. But it perhaps couldn't do the same if they're explicitly hiring entry-level people with no prior experience. But that's where a giant grey area develops.
The main thing to keep in mind is that basically none of these tests claim they're measuring IQ, and for good reason. The golden standard is for these tests to be relevant to the job and it is very hard to argue that something as generic as IQ is ultimately a job requirement. These tests usually focus on certain aspects of "intelligence" like reading comprehension, data analysis, numerical reasoning and so on. It's hard to prove that you need a savant-level IQ in your accounting department while it's much easier to claim that you need people who are "good with numbers". There's also the fact that full IQ tests take way too long and have little added value over partial testing. IQ is simply too generic a number and tests that simulate real-life work activities aren't just safer from a legal perspective, they're also more useful to gauge performance.
The fact that relevant and non-discriminatory tests are simply a good practice means that you don't hear much about companies breaching this section of the CRA. As long as you using a somewhat professional test that looks at required abilities and skills for the job and you're not selecting for a specific cultural, demographic or racial profile you should generally be in the clear. But as always in these cases it's best to leave this part of the hiring process to HR professionals or to work with established testing companies. Self-designed tests can sometimes insert bias or subconsciously filter for certain demographics, which is why large companies have standardised processes for this.
Employers can conduct tests on prospective candidates, to evaluate their skills, knowledge and experience relating to the job vacancy, to ensure that the successful candidate meets the requirements necessary to fulfil his/her duties, just as certain qualifications are needed to work in some industries and/or job roles. This way, I think IQ related questions can also be included in such tests.
However, I do not know whether it's legal or not to conduct such tests based solely on IQ, therefore cannot comment on the legal aspect of this.
You should consult with your area's legal constitution or a legal consultant (maybe a legal consultant provides advice on this website) on employment and candidate selection processes to get a better understanding of the legal aspect of this.