The aim with this essay is to research how changes in marginal taxation of capital, changes households’ investment behaviour. To be able to do so, the introduction of investment savings accounts, or ISK, in Sweden will be studied. This since the ISK can be considered a tax-subsidised investment form, and as such could provide insight of how the households change their investment patterns when their marginal tax of capital is reduced. To be able to assess whether this was in fact the case, the method difference-in-difference will be used. Due to many similarities in tax systems, Denmark is chosen as a reference group, to the treatment group Sweden. The result from the two difference-in-differences shows that the ISK taxation had increased the total value of the households’ stock ownership, but that no significant change in the number of private stock owners could be observed.