G. Stanley Hall, answered from the texts and cited to the page.
Man and woman are not two variants of one type but two types, deep and complementary, differing in body and in soul from the ground up; and an education that ignores this, that trains girls exactly as boys and presses them into the same molds and rivalries, works against nature and injures the very womanhood it should ripen.1 The girl at adolescence is governed by a physiological periodicity and is preparing, whether she knows it or not, for the supreme function of motherhood, on which the future of the race depends.
To subject her in just these critical years to the identical strain, competition, and sedentary bookishness we impose on boys is to risk her health, her nerves, and her fertility for the sake of a false ideal of sameness. I would educate her richly, but in accordance with her nature and her destiny — for feeling, for beauty, for the home and the child and the community — rather than assimilate her to a masculine standard that unsexes without truly equaling. The differences are the glory of the sexes, not a defect to be engineered away.234
Is it possible that the lost year or two that we have been seek- ing so long all the way from the kindergarten to the university lies here, and that the girl's soul is taking a vacation while her body goes to school? Is life opening so fast for her and with such manifold new richness that school just now pales, like the moon before the rising sun of womanhood?Educational Problems Vol 2, p. 16
In many of the new fields opening in biology since Darwin, in embryology, botany, the study of children, animals, savages (teste Miss Fletcher), sociological investigation, to say nothing of all the vast body of work that requires painstaking detail, perseverance, and conscience, woman has superior ability, or her very sex gives her peculiar advantages where she is to lead and achieve great things in enlarging the…Adolescence Vol 2, p. 635
One of the most important factors in the problem of woman is how her periodicity has been regarded. Man has regarded her course as a badge of inferiority. He has turned from her, isolated her, called her unclean, thinking her for a time diseased and perhaps even infectious and sometimes enforcing fantastic regimen and taboos. She must withdraw, hide and feel humiliated.Educational Problems Vol 1, pp. 460–461
Remarkable, indeed, if any such there be, who go through all these long and intricate processes of transformation under the eyes of our precocious American schoolboys unscathed, for it is one of the most trying ordeals which womanhood has ever been called to face in all her history. The entire history and tradition of her sex from savagery up points to segregation with periods of isolation from prying male eyes.Educational Problems Vol 1, p. 462
Hall's account of sex differences and his opposition to identical higher education for women stood in the hereditarian line of Edward Clarke's Sex in Education (1873) and was used to resist women's advancement in education; it is now repudiated on both empirical and moral grounds. The central empirical claim — that rigorous coeducation damages women's health and reproductive capacity — was false and was refuted by the flourishing of the women's colleges and by direct study. Modern psychology finds sex differences in cognition to be small, heavily overlapping, and environmentally shaped, not the categorical two-types division Hall asserted, and the "periodicity" argument has no basis as a bar to education. This is among the positions where Hall's science tracked the prejudices of his era.