Galton recognised the limitations of his methods in these two works, and believed the question could be better studied by comparisons of twins. His method envisaged testing to see if twins who were similar at birth diverged in dissimilar environments, and whether twins dissimilar at birth converged when reared in similar environments. He again used the method of questionnaires to gather various sorts of data, which were tabulated and described in a paper The history of twins in 1875. In so doing he anticipated the modern field of behaviour genetics, which relies heavily on twin studies. He concluded that the evidence favoured nature rather than nurture. He also proposed adoption studies, including trans-racial adoption studies, to separate the effects of heredity and environment.
In 1875, Galton started growing sweet peas, and addressed the Royal Institution on his findings on 9 February 1877. He found that each group of progeny seeds followed a normal curve, and the curves were equally disperse. Each group was not centered about the parent's weight, but rather at a weight closer to the population average. Galton called this reversion, as every progeny group was distributed at a value that was closer to the population average than the parent. The deviation from the population average was in the same direction, but the magnitude of the deviation was only one-third as large. In doing so, Galton demonstrated that there was variability among each of the families, yet the families combined to produce a stable, normally distributed population. When Galton addressed the British Association for the Advancement of Science in 1885, he said of his investigation of sweet peas, "I was then blind to what I now perceive to be the simple explanation of the phenomenon."