What the previous post failed to point out was that Mayella was not free to speak the exact truth since her father was present in the courtroom. Mayella knew that if she answered Atticus's questions honestly, then she would have to face the wrath of Bob Ewell once she got home. There is little doubt that Bob had beaten his daughter before, and he may have even taken sexual liberties with her as well. But she knew that she could not tell the court that her father had beaten her or that she had kissed Tom for fear of the certain reprisal she would encounter from her father after the trial.
In the book To Kill a Mockingbird Mayella Ewell lived during a time when she would have had few rights in a court of law against her father acts. When she testifies in court she has the opportunity to tell the truth. Verbally and physically the chance exists.
Now one must look at the other side of Mayella's life. She lives with a drunk and violent father who was capable of beating and raping her. She is he older child caring for her siblings. We do not know how much she loves her siblings. She is uneducated, has no money, and no prospects for a boyfriend let alone a husband. She is dependent on the drunkard, her father, and afraid of him. She has been raised in ignorance and taught to back her father. Even when he has committed the most horrid of offences towards her, she lies. I am also doubtful that his rape was the first time that he had sex with his daughter.
Now we come to the nature of philosophy. A human being has the ethical responsibility to tell the truth to protect the life of another person. Could she have told the truth? Yes, she could have told the truth. There are many people who rise above he situation that she was in to tell the truth in far more desperate situation. In the end the choice was hers.