Galvanic skin response – it all sounds very technical, and maybe something a doctor might talk about, but in the world of private investigation, galvanic skin response, or GSR, is a crucial tool in discovering the truth.

You’ve probably seen footage of lie detector tests, where a machine analyses the amount of stress in the interviewee’s voice. More stress usually indicates that the interviewee is telling a lie. GSR is a little bit like that, except this time, they’re not listening to your voice – they’re listening to your skin. A subject would be connected to pads which are attached to the fingers and “listen” for the way your skin is responding to the questions being put to you.

GSR goes back to our caveman days, and works on our basic fears. Imagine the scenario for your caveman ancestor, hunting down that sabre toothed tiger. The anticipation and fear is making their body react with the fight or flight response. Their head is full of excited thoughts, wondering if they’re going to catch their prey, their heart is pounding, their skin starts to sweat a little, their hands go clammy, and chills travel down their spine. It’s these basic human responses which GSR is looking out for. GSR controls the amount of sweat which each part of the body produces. There’s anything between 200 and 600 sweat glands for every square centimetre of the hands and feet alone. 

Typically, humans associate sweat with the body’s natural attempts to cool down when it overheats, with the evaporation of sweat producing a cooling effect. However, the sweat glands in the hands and feet actually respond just as well to emotional stimuli, such as fear or excitement. The more emotional the person, whether that’s in a positive or negative way, the higher the GSR level.

Being a natural process which a human cannot initiate themselves, GSR is a product of your brain’s sympathetic activity. Thanks to this, it’s a very difficult test for interviewees to get around if they plan on lying during an interview.

A useful tool in its own right, GSR is frequently combined with other techniques like facial expression analysis and heart rate to provide a fuller picture of what an interviewee means, rather than what they say.