As with many things in life, in order to draw helpful insights out of an observation, you must first understand the context. In analyzing nonverbal cues, context is one of the most important factors. Without it, a single gesture or posture can take on a variety of distinct meanings.
Picture a person with their arms crossed over their chest. When deconstructed, the act of crossing our arms over our chest is traced back to our earliest physical memories of being held by our parents when we were in discomfort or in need of some oxytocin.* As we progress in life, we may well shun the comfort of having our parents' arms around us, but we don't lose the need for a physical response to distress. We simply do it for ourselves.
Say you observe someone crossing their arms over their chest. In order to get to the why, context will help us. Though there are many reasons behind a crossed arm gesture, here are a couple common contexts to consider.
1) Arm-crossing in relaxed contexts.** If you see someone in the comfort of their own home, or surrounded by friends and they adopt a crossed arm posture, chances are, they have gotten into the habit of feeling comfortable in this posture. Many professional headshots have people in a crossed arm stance. Many people are uncomfortable and self-conscious in front of a camera. Crossing their arms can help provide them a sense of reassurance and confidence.
2) Arm-crossing as discomfort. Discomfort comes in many forms. If you are in a room that is on the chilly side, outside in the evening, or simply wearing clothing that is not climate-appropriate, you will likely hold your arms closer to your chest - and in many cases cross them. It just so happens that temperatures in office spaces and conference rooms are often cooler than comfortable for many people, particularly women who also tend to wear lighter clothing than men. Holding your arms close to your ventral area is an efficient way to keep warm - crossed arms in this circumstance is a sign of physical discomfort - not necessarily an adverse reaction to what is being said at the time. If however, an individual starts a conversation with an open posture, and only at a specific point in time crosses their arms - this is something to pay attention to. What happened that made them change their posture? What is the tone of the conversation? Is someone getting chastised or attacked? Crossed arms in this situation are likely a reaction against a feeling of vulnerability or overexposure - it is in effect, a barrier.
As you can see with even this simple example, context is key. Once you have identified a posture, take the time to assess other contextual elements. These will be crucial to unlocking a deeper understanding of what someone may be experiencing at any given moment. Most importantly, don't jump to conclusions - take in the whole narrative when listening with your eyes.
*Desmond Morris - People Watching
**Joe Navarro - What Every Body is Saying