After attending primary school, he went to Amsterdam to study at the Inrichting voor Onderwijs in Koophandel en Nijverheid (Institution for Education in Commerce and Industry), with the intention of returning to help run his father's successful grain trading business.

[2] When he returned to Amersfoort, he formally broke with the church in 1869 and joined the freemasons, becoming master of the Haarlem Masonic lodge Vicit Vim Virtus in 1871.

His thought increasingly combined Douwes Dekker's thinking, as an atheist, feminist and freemason, with the philosophy of the English clergyman Thomas Robert Malthus and he came to be identified with Neo-Malthusianism.

[7] Once in office, he pushed for council minutes to be published and meetings to take place in the evening so that local people could attend.

[10] The group produced a radical manifesto including calls for universal suffrage, free primary education, separation of church and state, and the development of self-government in the colonies.

He was an advocate of municipal ownership of utilities, the vetting of companies for public contract on the basis of the payment of fair wages, and corporate responsibility for pensions.

[11] On 6 September 1899, Gerretsen was appointed an alderman in Amsterdam with responsibility for the care for the poor, trade institutions and the pension agency.

His reorganisation brought all medical, surgical and obstetric into public control, meaning doctors would now be permanent employees of the municipality.

In 1880, he met Aletta Jacobs, the first female doctor in the Netherlands since the enlightenment, who helped him understand the biology of birth control.

With Bernardus Hermanus Heldt and Jan Martinus Smit, he co-founded the Nieuw-Malthusiaansche Bond (Neo-Malthusian League or NMB) on 2 November 1881.

[15] He travelled extensively with his wife to promote these values, attending the 1904 International Council of Women and subsequently touring the United States shortly before his death.