Tax legislation can be tested by the courts against higher standards, including fundamental rights. Part of that test consists of the proportionality test. Here, traditionally, e.g. by the Constitutional Court, but also by the European Court of Justice, a very wide discretion is left to the legislator. The reasoning is that the legislator has a very wide margin of appreciation to formulate economic and social policy. Only in the case of a "manifestly unreasonable" measure, according to these courts, will the law be legally censured.

The question arises where the boundaries of "manifestly unreasonable" lie and whether the courts should take into account the advancing scientific insights, including in the field of the theory of optionality. To date, this does not appear to be the case.