A new study finds it's easy to figure out someone's identity based only on where they go with their smartphone, as it's tracked by GPS.
Researchers from MIT and elsewhere tracked about one-and-a-half million people for 15 months.
They determined it only took four "data points" to figure out a person's identity 95 percent of the time.
Data points include locations such as work, the gym, church or a coffee shop.
The researchers found patterns of our movements, when traced on a map, create something like a fingerprint that is unique to every person.
The findings have privacy advocates concerned.
And the study's authors say keeping data anonymous is not necessarily enough to ensure real privacy.
They say the findings represent fundamental constraints on a person's privacy.