Branching and the like (/branch, /skip) works at the <surveypage> level, not at the level of individual question elments <e.g. <radiobuttons>, <checkboxes>, etc.). In your case you'd have one <surveypage> that contains the gender question element. Then /branch to a different <surveypage> containing the follow-up questions if a subject answers 'female':
<surveypage demographics>
/ questions = [1=gender; 2=age]
/ branch = [if(radiobuttons.gender.response=="female")surveypage.femalefollowup]
</surveypage>
<surveypage femalefollowup>
/ questions = [1=stuff]
</surveypage>
<radiobuttons gender>
/ caption = "You are"
/ options = ("female","male")
</radiobuttons>
<textbox age>
/ caption = "Your age"
</textbox>
<textbox stuff>
/ caption = "You should only see this if you indicated 'female':"
</textbox>
<survey mysurvey>
/ pages = [1=demographics]
</survey>
Alternatively, you can use the /skip attribute. I.e. if the subject answers "male", skip the follow-up page:
<surveypage demographics>
/ questions = [1=gender; 2=age]
</surveypage>
<surveypage femalefollowup>
/ skip = [radiobuttons.gender.response=="male"]
/ questions = [1=stuff]
</surveypage>
<radiobuttons gender>
/ caption = "You are"
/ options = ("female","male")
</radiobuttons>
<textbox age>
/ caption = "Your age"
</textbox>
<textbox stuff>
/ caption = "You should only see this if you indicated 'female':"
</textbox>
<survey mysurvey>
/ pages = [1=demographics; 2=femalefollowup]
</survey>
Regards,
~Dave