The annual review of the world’s patent filings always tells an interesting story.

Some 2.9 million applications were filed in 2015, up 7.8% year-over-year. China led with 1.01 million filings, followed by the US (526,000) and Japan (454,000), reports the World Intellectual Property Organization.

But … (when it comes to China there’s always a big but) … only 4% of China’s applications were outside their own borders, while 45% of US applications were filed abroad.

Computer technology (7.9% of the total) saw the highest percentage of published patent applications worldwide, followed by electrical machinery (7.3%) and digital communication (4.9%), WIPO reports.

WIPO doesn’t indicate why Chinese inventors are by and large choosing only to protect their claims in-country. Here are some possible reasons:

1. The US requires that inventors obtain a “foreign filing license” before filing foreign patent applications on inventions that occur in the US.  “This allows the government to assess, for example, whether the technology could threaten US national security,” says Dennis Crouch, a professor at the University of Missouri School of Law and co-director of the Center for Intellectual Property and Entrepreneurship.

2. China, on the other hand, requires inventors to first file domestically, where it will then determine whether the invention needs to remain secret for security or other purposes. Only then is the inventor allowed to submit an application abroad.

In summary, domestic firewalls in the world’s two largest markets could well be hampering outsiders.