SPOILER ALERT!
~ Anxiety is characterized by worry and avoidance. Fear is characterized by thoughts of imminent danger and escape
~ Both anxiety and fear include physiological reactions, BUT nervousness and inability to relax is specific to anxiety and intense bodily symptoms like heart racing exist only in fear.
~ In anxiety, attention to threatening cues is on an unconscious level, but in depression it is a strategic response, something that was well-thought out.
Agreeing on what the unique characteristics of an anxiety disorder prove to be a difficult task for experts working on the DSM V, which is the diagnostic bible in psychiatry. The difficulties arise when attempting to distinguish anxiety from fear and other mood disorders, as they share some internalizing factors.
Anxiety vs Fear
The main distinguishing characteristic between fear and anxiety is the temporal component. Indeed, anxiety is a future-oriented state associated with mental preparation for possible negative events, while fear is a response to a present and imminent danger.
However, looking at the three types of symptom-response observed in all psychiatric disorders reveal some similarities between them. Those responses include a verbal component (i.e., thoughts), a behavioral component (i.e, actions), and a somato-visceral component (i.e., bodily symptoms).
For example, anxiety is characterized by worry (verbal component), avoidance (behavioral component), and muscle tension (somato-visceral component). By contrast, fear symptoms include thoughts of imminent danger, escape and acute autonomic arousal such as sweating, trembling and heart palpitations.
Although those symptoms appear distinct, they actually lie on a continuum where they sometimes merge and diverge at varying degrees. For example, some of the somato-visceral symptoms common in fear also appear in anxiety, such as sweating, heart palpitations, and shaking.
There is nonetheless evidence supporting a clear distinction between fear and anxiety for two specific symptom-response. The first one relates to symptoms of physiological arousal that are specific to fear (for example, heart racing) and the other one involves subjective anxiety with symptoms like inability to relax and nervousness that are only reported for anxiety.
Anxiety disorder vs other mood disorders
In a similar vein, distinguishing the features of anxiety disorders from those of other mood disorders is another subject of contention among the experts. Particularly, the characteristic sensitivity to explicit threatening cues (e.g., an angry face) and safety cues (e.g., a smiley face) in anxiety disorders is also seen in other mood disorders.
In fact, there is not sufficient data to ascertain whether this hypersensitivity is exclusive of a particular anxiety disorder (e.g., post-traumatic stress disorder) or whether it is exclusive to anxiety disorders relative to depression.
In addition, it is not clear whether stress reactivity (i.e., physiological arousal to aversive stimuli like a crowd in agoraphobia) differs between anxiety disorders and mood disorders. There is however indication that elevated anxiety in anticipation of generic threat is characteristic of post-traumatic stress disorder (PTSD) and panic disorder (PD), while elevated acute fear is more prominent in PTSD (disorder specific threat such as seeing the place where a trauma took place) and specific phobia.
Lastly, there is evidence supporting cognitive differences between anxiety disorders and depression, albeit at specific degrees. For example, attention bias to threatening cues (i.e., reacting to an angry face or an emotional word) is observed in both types of disorders, but it happens on an unconscious level in anxiety disorders, while in depression it is more of a strategic, well thought-out response.
Reference:
Craske MG, Rauch SL, Ursano R, Prenoveau J, Pine DS, Zinbarg RE. What is an anxiety disorder? Depress Anxiety. 2009;26(12):1066-85. doi: 10.1002/da.20633. PMID: 19957279.