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The Filter Theory of Mate selection says that finding a mate works like this:

Pool of all possible mating partners

		|
		|

\      propinquity filter         /
 \     attractiveness filter     /
  \    social background filter /
   \   consensus filter        /
    \  complementary filter   /
     \ readiness filter      /
      \                     /
	__________________
                |
	     A mate

The Propinquity Filter
This one's first and most important - you have to be able to meet your potential mate before you can even think about gettin' it on.

The Attractiveness Filter
This one's only slightly less important, but it's a little wider because different things attract different people (see You are smart, I would like to make love to you vs. men who judge women by their looks). In general, males tend to value physical attractiveness more and females tend to value economic earning power more.

The Social Background Filter
If you're this far, you're halfway there. This filter tends to bring together people similar in religion, political affiliation, education, occupation, social class, etc. This one's not absolute and is fairly easily bypassed, but doing so makes the next filter doubly important.

The Consensus Filter
This filter brings together couples of similar attitudes and values. Also, if the Social Background filter was not met, couples generally have to come to a consensus about whatever they didn't have in common - for instance, agreeing (overtly or not) to not discuss divisive political matters. If this is not done, the relationship is likely to be very unstable.

The Complementary Filter
To pass this filter couples must complement each other (complete each other) and provide a balance for the relationship - this includes couples who are in love.

The Readiness Filter
And to pass this, final, filter, both members of the couple must be ready to settle down, make babies, whatever - the "time is right". Males generally tend to take longer to get past this filter than females.

And if you've gotten this far, well, you've found yourself a mate. Why are you still reading? grin.