“What makes Iago evil?” Joan Didion asks, opening her novel Play it As it Lays.
The more interesting question, to my mind, is what makes Desdemona – or indeed anyone — good?
Every story that we are told ingrains in us the idea that we were born to be selfish.  Left to our own devices, without the taming influence of a religion or social contract, we would act according to our true natures, which is to say meanly cold-bloodedly and entirely for self-preservation.
As the philosopher Thomas Hobbes once put it, altruism is an illusion; anyone appearing to act unselfishly did so simply ‘to deliver his mind from the pain of compassion.’ According to this mindset, we do nice things, basically, out of guilt; a kindness is essentially hypocrisy.
Altruism and evolution
I’ve been thinking about this after reading about an eccentric American scientist called George Price, who became obsessed with the origins of kindness.  He could not square altruism with biology – specifically evolution as it was understood at the time.
As he thought about it, altruism was completely at odds with Charles Darwin’s notion of survival of the fittest.  Acting unselfishly out of a concern for the needs or interests of others, regardless of the personal consequences, is potentially deleterious to the self; in fact, helping another in a tight spot can also reduce your possibility of survival. 
Altruism, one could argue, is ultimately an act of purposeful self-destruction. In a zero sum game, it is deliberately choosing the shorter straw. 
Game theory
To answer this question, Price moved to London and sought out the evolutionary biologist W. D. Hamilton.  Together they worked out a mathematical equation for altruism, using game theory to work out how altruism aids evolution.
Their theory explains altruistic behavior from the perspective of the gene and its need to replicate. The gene that promotes the desire to help other family members aids its own replication because it will help the spread of individuals bearing copies of that gene. In other words, birds will feed the young of relatives in order to increase the number of their shared genes in future generation.
The other face of this theory is the ‘helper’ idea –that an animal that offers itself as a helper increases its own chances of survival and future reproduction or ensures the survival of the family. For instance, the white-fronted African bee eater will fight snakes and other predators, forage for food for its relatives and put off having its own young to help its close relatives raise their own young. Helping is also a means of propagating the gene.
A variation on this idea is ‘group selection’ or group adaptation, or operating on behalf of the group’s gene pool.  This is thought to occur because most of the members of the group, as with bees, are ultimately related to the queen. All of the worker bees are deliberately sterile and forfeit their reproductive fitness because the bees within any particular colony are mostly relatives of each other and so are simply hardwired by a biological evolutionary imperative – to ensure that queen survives in order to pass on their genes. 
Finally, there is the ‘I’ll-scratch-your-back theory’:  a chimp will groom another chimp only because he’s fully expecting reciprocity. Richard Dawkins even offers a 'cost-benefit' equation that calculates the point at which it becomes genetically advantageous for an animal to display altruism.
Random acts of kindness
The ultimate problem with the theory is the vast number of exceptions to the rule.  Extensive studies of animals offer countless instances where animals are capable of random acts of extraordinary self-sacrifice, compassion, courage and generosity toward members of their own species, members of other species and even toward humans, often to their own detriment.
Many species of animals employ alarm and information systems about danger and food, even when this would endanger them.  Ververt monkeys use alarm calls to warn other monkeys of impending attack, even though raising the alarm could increase their own chances of being harmed.
Animals also routinely evidence moments in which they put aside the most fundamental drive of all: the need to eat. In innumerable instances, animals have shared food or ensured that weaker individuals in a pack or herd be fed, even if it means giving up their own food. This occurs even in species like red foxes, known for jealously guarding of their own catch.
Animals have been known to protect and share food with the injured or less successful in a pack – again, in situations where they themselves may face harm. In one instance Tatu, a mongoose, whose paw had been injured in a fight was unable to fight. The other mongooses in her pack began foraging close to her, so that she'd have more food and even gave up some of their daily food to her.
The Dawkins'-eye view of the universe would argue that altruism is impossible among animals that are not closely related as it runs counter to survival. This reductive argument also falls down in the face of many examples that describe assistance and cooperation by unrelated animals for no apparent self-serving purpose.
Friendly chimps
It is now possible to argue otherwise now, thanks to technology that can analyze fecal samples and from the DNA determine which animals are related to each other. This technology served a recent German-American field project of primatologists in Kibale National Park in Uganda studying the social relations of entire colony of chimpanzees. 
From an exhausting and messy analysis of the excrement gathered in the chimp’s grounds as they went about their days, the scientists were able to determine that although chimps spend more time with their relatives, they also are highly cooperative among those with whom they lack any sort of genetic kinship.  Kevin Langergraber and his colleagues concluded that ‘males in the majority of highly affiliative and cooperative dyads are unrelated or distantly related.’ And this is among highly competitive chimps.
Explaining altruism requires acknowledging a quality in living things that drives stake into the very heart of biology as we know it.  It’s more likely that we weren’t born to be selfish at all, but born to be good. 
Maybe it selfishness is the real pathology — bred into people who grow up in a pathological environment — and goodness is hardwired and abandoned out of cultural necessity in our eat-or-be-eaten society.
If this is true, we need another view of how evolution works.
We see this at its purest with animals.  An animal acts as it must,without a new set of cultural rationalizations (such as our most recent one: greed is good).  They help to remind us who we really are.