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