sex derivative: characteristic of male/female differences that are secondary or subsidiary to sex-irreducible, primary differences, and are for the most part under the influence of sex hormones.

Dictionary of Sexology Project: Main Index

The derivative of sex can be expressed as a linear function reflected across the X-Axis. Depending upon the participants (and their skill), it could be any variation of the following expression:

Y` = -X

That's what it should be, anyway. An unforunate (and often embarassing) scenario is when one member of the party is somehow incompatible, resulting in a warping of the original F(X) equation into something other than a concave-up parabolla. In this case, the sex derivative may appear as a sheepish horizontal line at Y = 0.

Don't let this happen to you. Make sure your sex is both inverted and exponential.