What are the motifs in The Tortilla Curtain by T. Coraghessan Boyle?

Borders are a recurring idea in the book. The novel contains many physical borders that separate Candidó and América from their rich neighbors. The largest border is the one that they have already crossed, the border that separates the United States from Mexico. No one in the novel can alter the presence of that border, yet the residents of Arroyo Blanco Estates choose to erect the novel's other borders.