Tom Christiansen brings to my attention a new step in camera technology: smile-detection software.
This brings a topic almost full circle:
The Expression of Emotion in Man and Animals, by Charles Darwin, talks about the biology of things like facial expressions.
You can look at Darwin's books as his exploration of different ways to reason about deep time -- timespans longer than you can observe in the laboratory. How do you figure out how soil is formed, a process that takes place over decades? See his book on earthworms. Coral reefs? See his book on, well, coral reefs.
How can you separate out learned from innate in facial expressions? Darwin notices that blind people smile and frown. Clever dude. There's much more, but, again, see the book.
Not only does the book include written reports of interesting data, insightful reasoning, and biological illustrations, but he brings in new technology in the service of science: 19th-century photographs of people smiling.