Superman buffs commonly hope to escape this kind of partiality by assuming: that what is needed is simply more of a single, timeless abstraction, intelligence, measured by intelligence tests; and that that abstraction is a genetically distinct characteristic, controlled by its own gene or genes. Both these ideas, however, are indefensible. There is a systematic distortion of language in the frequent talk of genes as 'coding for' behaviour, as if a given gene produced a given unit of behaviour, independent of all experience, as infallibly as it might produce brown eyes. Most properties are affected by many genes; most genes affect many properties. The effect of a given gene can vary greatly, too, according to the influence of other genes which are combined with it. Moreover, genes tend to be correlated in blocks in a way which usually makes it hard to identify the influence of each of them.