Throughout the Will Trent series, Karin Slaughter explores the concept of family through Will’s complicated history and relationships. This Is Why We Lied focuses particularly on the evolution of Will’s family and contrasts it with the McAlpine family. In doing so, the novel explores the definition of family and what makes a family successful.

Mercy McAlpine is a mother, and both Will and Faith connect with her because of it. Like Mercy, Will’s mother had him when she was quite young and struggled with substance use disorder and a legacy of abuse. Faith feels a connection with Mercy even after her death, reflecting that “Mercy had been a mother. Not just a mother, but a mother like Faith. They had both given birth to sons when they were barely more than children themselves” (257). The only difference between them that Faith can see is that “Faith had been lucky that her family had supported her. Without their strength holding her up, she could’ve just as easily ended up like Mercy McAlpine” (257). Faith admits that she and Mercy seem to be attracted to the same kind of man when she describes Dave as a “reprehensible abuser” who was good-looking “in a bedraggled, high-school-prom-gone-to-seed kind of way.