What is the time-distance decay effect in diffusion?

• A. The rate of diffusion increases with distance.
• B. The likelihood and rate of diffusion decrease with distance and time from the source.
• C. Time has no effect on diffusion.
• D. Diffusion only occurs after a long time delay.

Answer: (B)

The main idea here is that diffusion gets weaker as you move farther from where it started and as more time passes since the diffusion began. Interactions and contact between places aren’t the same over all distances; the farther apart two places are, the less likely it is that a trait, idea, or innovation will jump from one to the other quickly, simply because people, channels, and opportunities to spread are thinner with distance. At the same time, diffusion is a process that unfolds over time, and the strength of its spread tends to diminish as distance grows the farther you are from the source. In other words, this kind of diffusion has a stronger, faster impact nearby and a slower, weaker reach as you look farther away and further along in time. That’s why the statement that the likelihood and rate of diffusion decrease with distance and time from the source best matches how diffusion actually works.