The play’s intimate look into the lives of the Hunsdorfer family illustrates universal truths about the human struggle for acceptance and self-efficacy but reveals that each person adapts and responds differently to life’s harsh uncertainties. The play demonstrates that some people will thrive while others will barely survive under similar adverse conditions, much like the marigolds in the parallel world of science. For Tillie, science generates life and hope amid the despair of Beatrice’s self-imposed isolation.

Neither Beatrice, with her cynical withdrawal, nor Tillie, with her naïve optimism, presents a healthy solution for dealing with an imperfect world, yet each represents an authentic, if somewhat extreme, coping mechanism. By choosing to see only the potential good in atomic science, for example, Tillie triumphs over her mother’s fear and pessimism but is blind to the dangers of radioactivity. Both Tillie and Beatrice have been mocked and treated as social outcasts, but Tillie, who does not value the opinions of outsiders, cannot be wounded by their disdain. Beatrice and Ruth, on the other hand, place inordinate importance on the opinions of others and thus feel compelled to compensate for this perceived loss of esteem, the result of social rejection, by inflicting cruelty upon Tillie and each other. Beatrice and Ruth allow the negativity of others to defeat them, and it is their attitudes that seal their fate.

The numerous reminders of decay that pervade the Hunsdorfer lives reinforce a related theme, the inevitability of death. Nanny’s absence of understanding and physical decline mirror the chaotic disarray and faded spirit of the house. Beatrice takes the life of the rabbit to destroy any hope that manages to survive in her daughters. At times of great stress, Ruth becomes paralyzed by seizures. Amid this erratic dysfunction, science, with its order and insight into the origins of the universe, becomes Tillie’s lifeline, connecting her to a larger world from which Beatrice, in fear and bitterness, hides. Only Tillie’s marigolds, and through them, Tillie herself, experience vitality and growth. Tillie alone has the courage and stamina to pursue her dreams, and this is the spark that empowers her. This optimism reinforces the central idea of the play, since Tillie’s positive outlook lifts her dreams beyond those of Beatrice, whose tea shop will never be achieved. Just as some marigolds, when exposed to the cobalt-60, yield spectacular double-blooms—symbolic of Tillie—others mutate into nonvital organisms, like the forever frustrated Beatrice and the shallow, unkind Ruth.

Triumph in the Face of Adversity
The characters in The Effect of Gamma Rays on Man-in-the-Moon Marigolds all face adversity, but each reacts very differently. Beatrice has allowed the difficulties and bad luck she has encountered throughout life to defeat her. She attempts to better her life, but her bitterness presents a barrier. Beatrice is so caught up in the negative, unfair aspects of life that she is unable to see any goodness around her. Tillie, however, is able to prevail, even in the worst circumstances. She can find beauty in the smallest detail. No matter how many times she is chastised or disappointed, she gets back up and tries again. She is a survivor.

Self-Image
Beatrice and Ruth are very concerned with how they appear to others. Ruth is constantly worried about how she looks. She wears tight sweaters and refuses to go to school without first putting on makeup. Ruth wants to fit in and is very fickle in her relationship to Tillie. Most of the time she considers Tillie an embarrassment and doesn't want to be associated with her. Ruth quickly changes her mind, however, when Tillie wins the science fair, because she wants to boost her own image by bragging about Tillie's accomplishments. Beatrice is also trapped by a need to fit in. She talks about how popular she was in high school, but eventually the audience discovers that this wasn't true at all. Beatrice was an outcast in high school who was constantly teased. This caused her a great deal of pain and was a factor in her withdrawal from the world. Tillie is the only one of the family who is secure in her self-image. Although she is teased and made fun of, she continues to be true to herself. She doesn't try to change and fit in with the crowd, but instead pursues the things that are important to her. This ultimately leads to her success in the science fair, and the playwright suggests that it will help Tillie succeed in life.

Dreams
Dreams are a very important theme in The Effect of Gamma Rays on Man-in-the-Moon Marigolds. Beatrice, Tillie, and Ruth all share their dreams or tell of dreams they have had at some point in the play. In Act I, Ruth has a seizure brought on by a nightmare. Beatrice also relates a recurring nightmare about her father and his vegetable wagon.

Throughout the play, Beatrice is constantly talking about her dreams of opening a tea shop, of becoming a dancer, and of escaping her dreary existence. Tillie's dreams are sparked by her discoveries in science class. Because Tillie can still find good in the world, the possibility exists for her dreams to come true.

Life versus Death
Images of death and decay are prominent in the play. Beatrice kills most things around her. She chloroforms the rabbit, and she tries to kill her daughters' spirits by constantly berating and belittling them. Nanny is nothing more than a walking corpse. Even the room looks as if it is decaying, with its piles of newspaper and objects strewn everywhere. Tillie is the only one who connects to a life force. She plants the marigold seeds that eventually grow into many strange and wonderful mutations.

The Inability to Make Meaningful Human Connections
Just as Nanny is shut out from the outside world through hearing loss and the thick cataracts that cover her eyes, Beatrice is shut out from the outside world through her fear. She has covered the large window of the front room with newspaper so passers-by cannot see in. She doesn't want the family to interact with the outside world. Beatrice is just as trapped within her own self-made prison as Nanny is within her aging and failing body. Also, Beatrice, Ruth, and Tillie are unable to truly connect and share with one another. Most of the communication between Beatrice and Ruth consists of yelling and bickering, while Tillie chooses to remain silent. None of the three is willing to really open up and share their true feelings with their family members.

Half-life
The half-life of the radioactive isotopes that Tillie explains to Beatrice symbolizes numerous things about the family. Beatrice uses the words "half-life'' literally to describe the unfulfilled potential she feels in her own life. Also, just as the radioactivity will go on forever, so will the unfortunate situations and bitterness that Beatrice is caught in. Half-life also symbolizes hope for Tillie, however. She relates to the concept in a positive way, recognizing the magic potential of something that never ends. To Tillie, half-life represents new areas just waiting to be explored, filled with wonder and fantastic possibilities.