Is Pearl's behavior really unnatural for a child, or does Hester just imagine that it is?"The Scarlet Letter" by Nathaniel Hawthorne

In "Scarlet Letter," Pearl is more of a symbol  that she is a real child. The reader may recall that in Chapter VIII she tells the Reverend Wilson at the governor's mansion that she

had been plucked by her mother off the bush of wild roses that grew by the prison door.

This allusion to the roses at the prison suggests the symbol of passion that was in Chapter I.  Hester reaffirms that Pearl acts as a symbol:

She is my happiness!--she is my torture, none the less!  Pearl keeps me here in life! Pearl punishes me too!  See ye not, she is the scarlet letter, only capable of being loved, and so endowed with a millionfold the power of retribution for my sin? 

This representation of Pearl as symbolic of the passion between Hester and Arthur Dimmesdale is underscored in ChapterXXIII as the three stand on the scafforld.

As this symbol, therefore, Pearl is truly an unnatural child.  She does, as Hester says, remind her mother of the sin.  When they are at the governor's mansion, for instance, Pearl delights in how large the scarlet letter appears when reflected in the suit of armor.  At play she surrounds the A on her mother's bosom with flowers.  When her mother casts off the A onto the other side of the brook, Pearl refuses to cross and return to her mother until Hester again dons the letter.

A perplexing mix of strong moods, Pearl is given to uncontrolled laughter at one moment and at another she is silent or fiercely angry with a capacity for the "bitterest hatred that can be supposed to rankly in a childish bosom."  In fact, her behavior is so unusual that she is often referred to as "elf-child," "Imp," and "airy sprite."  Of course, the child has not had a normal life since she is isolated with her mother.  Nevertheless, there is something that is "other-worldly" about this child of such a quick mind and intuitive nature.  It is not until the events of Chapter XXIII that Pearl is brought fully into the world of humanity as she kisses her dying father. 

Critics perceive part of Hawthorne's design to not attempt to develop realistically a normal girl.  For, throughout the novel, Pearl acts more as a symbol than as a mere child:  she is the symbol of the illicit union, a symbol of passion, the warfare of Hester's and of Dimmesdale's spirits, a symbol of torment for Hester, and the embodiment of Hester's conscience. As symbol she is Hawthorne's greatest contribution to American literature.