ISO 5218

ISO/IEC 5218:2004

ISO/IEC JTC1 SC32 WG2

http://standards.iso.org/ittf/PubliclyAvailableStandards/c036266_ISO_IEC_5218_2004(E_F).zip

2 Representation of human sexes
Human sex is represented by a one-character 
numeric code. 
The following data elements and codes are used: 

Data elements   Code 
Not known    0 (zero) 
Male     1 (one) 
Female     2 (two) 
Not applicable   9 (nine)