Gonadal hormones may thus become activated in the spring, but as a result of loss of sensitivity to light over the summer hormone levels may diminish as fall approaches. Despite these facts, day length is a powerful influence. Thus, steers increase their weight more rapidly in the winter when artificial light is supplied to lengthen the day. If pineal effects on sex hormone levels are important, then the birth months of lefthanders, and of those with learning disorders, might not be uniform throughout the year, since fetuses conceived at different seasons might be subjected to very different hormonal environments. These effects should differ in the Northern and Southern Hemispheres and at the equator, although other factors, such as variations in the ethnic composition of populations, would also have to be considered.
Data are still very sparse.
There are, however, influences in pregnancy that tend to diminish the extent of left-sided predominance, at least in the regions involved in handedness and language, and thus secondarily to result in larger regions on the right side. As noted earlier, our hypothesis is that some factor related to male sex, perhaps testosterone or some closely related factor, is the most likely candidate. The net effect of these intrauterine influences is to produce a shift from left predominance to symmetry, and in a smaller number of cases to modest right predominance. Since the effect of a gene is to play a role in some form of chemical reaction, it is not surprising that genetic determination is not absolute. Every chemical reaction can be modified by alterations in pressure, temperature, pH, light, the presence of other substances, the availability of chemical precursors, and the rate at which products are removed.