Statistics New Zealand looks to be first in the world to start collecting statistics on individuals' gender identity. And they're taking an eminently sensible approach to it.
If you only asked people with which gender they identified, you would mess up a pile of clinical applications where a standard diagnostic flowchart will depend on biological sex. You don't suspect prostate cancer for a bundle of symptoms if the person presents as a woman, but you might if she was born male.
From the FAQ:
How will asking for gender identity information affect me?
You may notice in the future that some government forms will ask for your sex and others will ask what gender you identify as. Other forms may ask you both. A question about gender identity will only be included when there is a clearly defined need for the information to be collected.
Why would some forms ask for sex and others gender identity?What are the categories?
In some situations knowing someone’s sex as recorded at birth is important. For example, in clinical situations when various medications react differently to each sex; or to calculate population growth. In other situations, knowing someone’s gender identity is important to ensure adequate services are provided, and that the individual is addressed correctly. Other times, both types of data may be required.
Longitudinal work in 10 years will be interesting: predicting transitions across categories, identifying effects of those transitions conditional on the factors predicting transition...
Classification of gender identityThe standard classification of gender identity is a hierarchical classification of two levels. Level 1 of the classification has three categories. Level 2 has six categories.
Classification of gender identity
11 Male / Tāne
21 Female / Wahine
- Gender diverse
30 Gender diverse not further defined / Ira tāngata kōwhiri kore
31 Transgender male to female / Whakawahine
32 Transgender female to male / Tangata ira tāne
39 Gender diverse not elsewhere classified / Ira tāngata kōwhiri kore
Update: In case you wondered, here's the Encyclopedia of New Zealand on gender diversity in Polynesian cultures.