Sexing Livebearers with Temperature Change - Possible??

My wife is a gecko person, and has learned that you can help precipitate the
gender by changing the temperature of the eggs of gecks....

I've had 3 batchs of guppies, at a rate of 85% male, 15% female... How can I
yeild more females, temp up or down???

This is just a test to see if it's possible, but I'm eager and albe to test
that theory if anyone wants to help hypothize...????

Right now I've got 3 pregnant females, by the same male, right now seperated
and with females only @ 79dF.

If I increase temp to 82dF, what will happen??
If I decrease temp to 74dF, what will happen??

Anyone can ring in on this...