The dentist was examining the patient’s broken tooth. “Oh, no,” he said. “It looks like that crack is in danger of propagating.”
“Mmmpf, mffppp,” the patient responded, her mouth full of cotton and dentist’s tools. What she meant was, “I thought propagate meant creating babies or reproducing, not just getting bigger.”
The dentist immediately turned to Google and pulled up dozens of sites in which “propagate” was used the way he had, and in other ways as well. “Propagate,” it turns out, means more than just reproduction, and not just birds and bees kinds of reproduction.
“Wave propagation” is the method by which waves of water, sound, light, or other things move; “crack propagation” is the movement of the front or tip of a crack through a material. The federal government even has a link to a study on “Crack propagation in teeth,” albeit with regard to death. And, of course, it means to reproduce, both with sex and without it.
As a verb, “propagate” can be transitive (needing an object to act on) or intransitive (acting all by itself).
In The Merriam-Webster Dictionary, for example, the first definition for “propagate” is the transitive, reproductive type: “to cause to continue or increase by sexual or asexual reproduction.” M-W’s third transitive definition is “to cause to spread out and affect a greater number or greater area,” or extend. In those cases, it needs an object: “propagate a species,” “propagate the crack.”
Intransitively, “propagate” means “to multiply sexually or asexually,” increase or extend,” and “to travel through space or a material—used of wave energy (as light, sound, or radio waves)” … or teeth.
Webster’s New World College Dictionary also likes the reproduction definition, waiting until the fifth definition of the transitive verb “propagate” before it mentions “to transmit (esp. sound waves or electromagnetic radiation) through a medium.”
“Propagate” comes from Latin words meaning “to reproduce,” as well as “to perpetuate, to prolong, to enlarge, extend,” the Oxford English Dictionary says. Its earliest uses, beginning in the 16th century, were “To produce (offspring),” the OED says. Sexually or asexually.
About 100 years later, the OED says, the first use of the transitive “propagate” appeared to mean “To extend the action or operation of; to transmit, convey, or spread (a wave, vibration, energy, etc.) in a particular direction or through a particular medium; to cause (a crack) to progress through a material.”
That’s exactly what that tooth crack was trying to do, before it was capped.
People often try to “propagate” their beliefs, through “propaganda.” Not surprisingly, they’re related: The OED traces the etymology of “propaganda” to a title meaning “congregation for propagating the faith.”
“Propaganda” has very negative connotations. One company’s “public relations release” can easily be seen as “propaganda” by a competitor or someone who does not agree with the contents. “Propagate” has gentler connotations: Most of the Nexis mentions of “propagate” in the past six months have to do with replicating plants or animals, not people (or lengthening cracks). “Propagate” is rare enough that you need to be sure your audience knows it, or give enough context for someone to figure it out.
One other difference: “Propaganda,” unlike “propagation,” seldom involves sex. It does, though, sometimes have a bite.