They make graphs to chart the prevalence of love over a lifetime...

(Source: Betsey Stevenson and Justin Wolfers).

... and segregated by income.

(Source: Betsey Stevenson and Justin Wolfers)

They build equations that, when graphed, will produce a heart. Via Incidental Economist:

They send Valentines as carefully crafted jokes about health policy ...

... and the Fed ...

... and political science too.

And, when all else fails, they write a poem -- accompanied by a graph, of course.