A good question, because some people question the validity of graphology for this very reason.
In the same way that your body language reflects your emotions and current state of mind, and the particular demands of any given moment, so too does your handwriting.
In the case of some people, these external influences, and their accompanying internal responses, have a very noticeable effect on the appearance of the writing, whereas in other cases the writing is far more resistant to change and maintains roughly the same appearance.
Each person’s writing, however, has a ‘core’ group of handwriting features that reveal the dominant forces and characteristics in the personality, and this remains largely the same irrespective of the degree of change in the appearance of the writing.
To give an analogy: sometimes the sea is very stormy, and then it looks completely different from when the weather is calm. However, even during a violent storm, the water deep below the surface remains largely unchanged. Similarly - (except in extremely rare cases when individuals are suffering from psychiatric personality disorders such as intermittent schizophrenia) - the periodic changes you notice in your own or other people’s writing which appear and disappear from the writing as mental focus, emotions and stress levels alter, usually represent only very insignificant changes in terms of how they affect the interpretation of personality . For instance, you can expect noticeable changes in the appearance of someone’s writing if they become temporarily depressed, fearful, angry or perhaps deeply absorbed in a particular activity.
So handwriting in general is in a constant state of flux, to a greater or lesser extent. However, although some believe that this means handwriting merely reflects the present moment, in fact, it is fundamentally shaped by the sum total of all past experiences – pleasant and unpleasant – which have made a deep impression on us. This is why, even if a person’s writing seems to alter very noticeably from day to day, or even moment to moment, it is only superficial temporary elements of the personality that are shifting and changing, because the core features of the handwriting, which are formed by the past, remain unaltered, and it is these which provide the expert graphologist with all the essential information pertaining to our fundamental nature. In this sense, a graphological profile is a window through which we can look at the effect on our personality and behaviour that has been exerted by the various events in our life that have conditioned who we have become today.