US, Sept. 16 -- The service sector of the global economy keeps growing in unexpected ways. Every big city has thousands of businesses that offer a wide variety of services. In Shenzhen, China, for example, you can walk into one store to get your hair styled, another store to get your nails polished, and a third store to get your body scratched.

Then you can go on a date with the cute person you met on Tinder, feeling confident that you have perfect hair and nails, and no itch to be scratched at an inopportune moment.

You may be wondering if an itch-scratching store actually exists in Shenzhen or if I'm just making things up, as I'm fond of doing. In fact, two itch-scratching stores exist in Shenzhen, the southeastern city near Hong Kon...