This thesis was written as part of a MOOC I took. More info here.
The Left Hand of Darkness is about duality in different forms, whether it be the duality of male-female, cold-warmth, or light-dark.
This concept is referenced clearly in a scene where Genly Ai draws the yin-yang symbol, then explains it to Estraven as “It is yin and yang. Light is the left hand of darkness… how did it go? Light, dark. Fear, courage. Cold, warmth. Female, male. It is yourself, Therem. Both and one.” The first half of that statement “Light is the left hand of darkness” is part of a Gethen poem which goes “Light is the left hand of darkness, And darkness the right hand of light. Two are one, life and death, lying, Together like lovers in kemmer, Like hands joined together, Like the end and the way.” The primary concept of Duality is that opposites complement each other, bring balance, and are an indivisible whole, inseperable from their parts.
The people of Gethen/Winter live with this duality in their individual bodies, and can physically manifest one or the other aspect of yin-yang — the male or female. Yet, they can maintain such a state only temporarily, before going back to their permanent state of neutrality. This is in stark contrast to humans, as, on a biological level, we actively seek for a mate to complete us.
Through the latter half of the novel we see that Ai is troubled constantly by the bitter cold of winter, whereas his companion Estraven seems unaffected by it. On the other hand, Ai is the stronger of the two, which Estraven admits to, in his diary. This shows how each of the two begin to overly manifest opposing characteristics when put together for long periods, and that this is a natural state of being. When Estraven goes in Kemmer during their march through the Ice, he is making their male-female interaction — and possible attraction — overt.
You can view the previously published theses here.