Your Smartphone; A Prosecutor’s Best Witness


Witnesses in the United States may choose to exercise their right to “plead the Fifth,” or refuse to answer a question because the response could provide self incriminating evidence of an illegal act. But how strong is this right if a prosecutor is already aware of where a witness was, what they saw, what they heard, and in some ways even what they were thinking at any given time on any given day? I do not refer to torture, truth serums, or the recent accomplishment of Berkeley scientists in creating visual pictures from brain waves. Rather, I refer to smart phones and the growing field of “reality mining” or “mobile phone sensing.”

The Progression of Mobile Phone Technology

Mobile phones had a humble beginning in 1973. Unlike modern iterations, the first mobile phone was bulky, difficult to transport, and had but one function; it allowed you to make a phone call. Technology has rapidly progressed from 1973, and today’s smart phones come rich with functionality through the use of embedded sensors. For example, the Apple iPhone comes equipped with an accelerometer, gyroscope, GPS, digital compass, ambient light detection, and dual microphones and cameras. These sensors give the iPhone the ability to speak, listen, locate, give directions, detect movement etc…  The new iPhone4S also responds to natural language queries.

“Mobile phone sensing” makes use of these digital senses to enable smart phones to become even more dynamic. Dartmouth College’s Smartphone Sensing Group
describes it as “turning the everyday smart phone into a cognitive phone by pushing intelligence to the phone and the computing cloud to make inferences about people’s behavior, surroundings and their life patterns.” The practical applications of this field seem enormous:

  • Researchers at Dartmouth college describe examples such as 1) using accelerometer data to automatically recognize different activities (e.g., running, walking, standing), 2) combining accelerometer data with location estimates from a phone’s GPS to recognize the mode of transportation such as walking, biking, driving, taking a bus or riding the subway and 3) continuously collecting audio from a phone’s microphone to “classify a diverse set of distinctive sounds associated with a particular context or activity in a person’s life, such as using an automated teller machine, being in a particular coffee shop, having a conversation, listening to music, making coffee, and driving.” Amongst a laundry list of other applications, this data can be translated into suggestions for how to avoid traffic, increase social networking, and for personal health care by tracking your physical activity or the number of times you’ve visited your doctor’s office.
  • An Oregon based technology company, Digimarc, has developed an application that uses a mobile phone’s camera and microphone to “detect digital watermarks encoded into magazines, newspapers, packaging and other printed materials, as well as identify songs in music and read QR codes.” Mobile phones can then recognize what it is that a user is looking at or listening to and make personalized suggestions.
  • Sandy Pentland (a leader in the field of mobile phone sensing and director of MIT’s Human Dynamics Laboratory) predicts that mobile phone sensing may be able to give clues to diagnosing depression by detecting changes in speech patterns, or using a phones motion sensors to reveal slight changes in gait, potentially acting as an early indicator of ailments such as Parkinson’s disease.

Smart Phone as a Witness

Yet these examples may not be the limit to what mobile phone sensing may be used for in the future. It seems possible that future criminal (or civil) trials may not need live witnesses so long as the sensor data from an accused individual’s smart phone was admissible as evidence. Before a word was said, it could be known where the individual was, who they were with, what they were looking at, listening to, talking about etc… Not just the day that a crime was committed, but days, weeks, or months in the past. These seem to be the personal experiences that individuals were traditionally allowed to protect to avoid self-incrimination.


The Fifth Amendment is concerned with what an individual must be witness to after a crime is committed, rather than before. Ex ante choices such as planning to commit a crime do not bar a person’s right to avoid self-incrimination. Thus a person who purchases a smart phone in order to utilize the advantages of mobile phone sensing (or for whatever other reason) should not diminish their rights under the Constitution. When we look ex post, the experiences that a phone interprets are the users, the senses that it has are digital extensions of the individuals. Those experiences and senses is what the Fifth Amendment is concerned with and should not be allowed as evidence of a person’s guilt.

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