Tenant Screening- Joint Income?

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It's common knowledge that a basic qualification for a tenant is to show income that is 3x's the rent. However, if a husband and wife or a boyfriend or girlfriend are filling out the application, is this a joint requirement or an individual requirement. 

Example: Rent is $1,000 per month.  

Does each applicant need to show that they earn a gross income of $3,000 or combined?