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Based on what you say, unless there are reasons for the lower pay which can be quantified and proven to exist fairly, there may be discrimination involved such as due to race, gender, age, etc. Not all cases where it seems discrimination is involved are necessarily due to discrimination, but where an employee believes they have been treated differently or given different benefits and opportunities due to their race, religion, age, gender, etc. they can consider filing a complaint with the United States Equal Enforcement Opportunity Commission here or here and/or speak to an attorney regarding a potential claim for discrimination and other potential claims under other state and federal laws.