The major themes in Stephen Karam's play The Humans include clashes of values between generations, infidelity and heartache, the difficult reality of Alzheimer's disease, and tensions between social classes. Let's look at these in more detail.
As the play opens, Erik and Deirdre are arriving at their daughter Brigid's home for Thanksgiving. Brigid lives with her boyfriend, Richard. Erik and Deirdre bring along Erik's mother, Momo, who is suffering from Alzheimer's disease. Also present is Erik and Deirdre's older daughter, Aimee, who has just broken up with her girlfriend.
We can begin to see the themes even in this brief summary. Deirdre and Brigid clash over religion and Brigid's unmarried state. Aimee is experiencing a heartache that she is struggling to cope with. Alzheimer's is front and center in the play in the form of Momo's violent outbursts. Tensions rise between Erik and Richard over social class as Richard is well-off, and Erik thinks he is just a “trust-fund baby.” Toward the end of the play, Erik reveals the secret of his infidelity. He has been fired from his job for having an affair with a coworker.
This is clearly not a happy family, and the play ends in near darkness, symbolizing the unresolved conflicts between the characters and the darkness of their lives.