By submitting a joint income tax return and signing the tax return, both spouses/partners declare their consent to joint assessment. Therefore, the joint tax return must be signed by both spouses/partners even if only one spouse/partner has earned income.
With your handwritten signature, you assume responsibility for the information provided in the tax return. In the case of a joint income tax return of spouses/partners, you are only responsible for the information that relates to yourself or the spouses/partners jointly. You are not responsible for information that concerns only the other spouse/partner.