As with the "time travel genetic disease" from the previous novel, Niffenegger takes her supernatural concept seriously and writes with far more precision and internal consistency than many science fiction authors do - in many ways this is a hard sci-fi ghost story. Elspeth's predicament becomes clear as she explores the parameters and boundaries herself. She takes a scientific, trial and error attitude to her new existence, which it is hinted at is somehow electrical in nature. Elspeth also gradually works out different ways in which she can or cannot interact with the others, and thus takes an increasing role in the proceedings. The author has considered fully the potential consequences of the ghost concept - an impossible love triangle that develops between Elspeth, Robert and Valentina is particularly poignant.
Martin's struggle to overcome his OCD after Marijke finally leaves him becomes a symbol for the central theme of the novel, control and letting go. Martin's condition also reinforces my idea that Niffenegger as a writer is particularly interested in mental illness and this is one way of reading The Time Traveler's Wife. The two pairs of twins depicted here are very much dominant and submissive pairs: interestingly they are physically "mirror twins" rather than "identical twins" - this has a bearing on their characters as Julia is healthy while Valentina has situs inversus (a problematic reversed heart.) We come to understand Julia's desire to care for Valentina and Valentina's desire to escape at any cost, and the dynamic between Elspeth and Edie, which is even more twisted, comes to light too. The characters are never simplistic or purely good or evil, for example Julia's overprotectiveness of Valentina is completely consistent with her tenderness towards Martin.
The book is also about love and how intense it becomes when the relationship is unequal. Towards the end of the novel many characters, driven to extremes by intense emotions, participate in some atrocities - with a surprising twist in the outcome, perfectly foreshadowed by earlier hints about the personality of one of the participants. Despite this, and as with The Time Traveler's Wife, the ending brings together both pain and hope.