The Taxation of Married Couples
The tax treatment of married couples has been a debating point throughout the existence of the income tax. Actual policies have varied over time and across countries. In recent decades there has been an international trend from joint to individual taxation of husbands and wives, such that the majority of European countries now use the individual as the basic unit of taxation. However, although the formal framework tends to be individual-based, the taxation of spouses is not separate. For example, it is common for tax laws to allow for the transfer of unused allowances between spouses, or to grant dependent spouse rebates or employment credits based on family income. In addition to these elements of jointness in the tax system, the eligibility for transfers tends to be related to family income rather than individual income. This applies to social assistance benefits and to family benefits like housing subsidies and child reliefs. These components combine so as create a complicated interaction between the earnings of one spouse and the taxes and transfers for the other. Quantifying the effective tax rates for husbands and wives in different countries is not an easy task.
You can read the Working papers below:
A Revised Efficiency Principle for the Taxation of Couples by Henrik Jacobsen Kleven & Claus Thustrup Kreiner 2004-09
The optimal income taxation of couples by Henrik Jacobsen Kleven & Claus Thustrup Kreiner & Emmanuel Saez, NBER WORKING PAPER SERIES, Working Paper 12685
Optimal tax and transfer programs for couples with extensive labor supply responses, Journal of Public Economics 95 (2011) 1485–1500,Herwig Immervoll, Henrik Jacobsen Kleven, Claus Thustrup Kreiner & Nicolaj Verdelin here