What is the main factor determining a tax return filing status?

The main factor determining a taxpayer's filing status is their relationship status. This includes whether they are single, married (filing jointly or separately), or a head of household. The relationship status provides critical context for the tax laws, as it affects the eligibility for various deductions, credits, and tax rates. For example, married couples may have the option to file jointly, which often results in a lower tax burden compared to filing separately. Additionally, a head of household status can provide a more favorable tax rate and higher standard deduction than filing as single, thus emphasizing the significance of the taxpayer's relationship status in determining their filing options.

The other factors mentioned—such as income size, the number of allowances claimed, and the amount of tax owed—are important in the context of overall tax calculations and obligations, but they do not directly dictate the filing status itself.