University of Otago academic Hamish Spencer with a copy of the new book he co-authored on the history of eugenics. Photo: Christine O'Connor.

It is a long-discredited theory, but a new book sets out to demonstrate the influence eugenics had in shaping New Zealand society.

Eugenics is a belief the genetic composition of the human race can be improved through selective breeding.

Today, for many, eugenics is automatically linked with Nazism. However, a century ago pillars of New Zealand society were prominent eugenicists.

Dunedin chief justice Sir Robert Stout was one and Plunket founder Sir Truby King made several statements supporting eugenic aims.

"I hope the book shows eugenics did play an important part in what happened in New Zealand," says Hamish Spencer, co-author of Eugenics At The Edge of Empire, being launched today.

"It played a role, even in some things we now think of as quite innocuous, such as the fitness campaigns of the 1930s ... which had eugenic overtones, even though they don't quite fall under the rubric of eugenics."

The first New Zealand branch of the Eugenics Education Society was established in Dunedin in 1910 at a meeting overseen by hospital board chairman J.H. Walker.

The Otago Daily Times devoted a large amount of space to the meeting.

"We must train the youth of both sexes to avoid mating with criminals, degenerates and the physically unfit," Emily Siedeberg, New Zealand's first female medical graduate, told the meeting.

St Matthews vicar William Curzon-Siggers spoke of parenthood being an instinct which should always be considered as having "a responsibility to the race", while Harry Bedford, a solicitor, former MP and "man of the world" looked to the "fit" not to shirk their "sacred obligation" to reproduce.

Prof Spencer said our objections to eugenics today were not to do with whether the theories worked or not, but were ethical or moral.

"We have to understand that eugenicists had laudable goals, they wanted to improve the human condition.

"It's the methods by which they wanted to do so which we find objectionable today."

Any state intervention in reproductive decisions today is anathema for most, but a century ago the notion of the state having a say on such matters seemed entirely reasonable to many.

"The other thing we find objectionable is an element of coercion in reproductive decisions," Prof Spencer said.

That was certainly a factor in the debate over law changes in 1928, when the Government proposed forced sterilisation (but not castration) of "mentally defective" persons, and barring the "mentally feeble" from marrying.

Several other countries and several American states already had similar legislation, and a fiery debate ensued in New Zealand before the government gave way and scrapped the sterilisation provisions from what became the Mental Defectives Act 1928.

Prof Spencer authored a chapter in Eugenics At The Edge of Empire on the history of the Act.

"Peter Fraser of Labour, in particular, led vehement opposition to the sterilisation and marriage prevention clauses," Prof Spencer said.

"The other large body of opposition came from the Catholic Church, which seems sort of obvious today when they have a history of taking views on reproductive decisions ... but the papal statement condemning sterilisation didn't come out until the 1930s."

Although the law did not permit it, evidence shows some New Zealanders were forcibly sterilised, some as late as the 1970s, Prof Spencer said.

While the arguments are now history, traces of eugenic thinking remain on the statute books in places such as marriage laws.

"Bans on first-cousin marriage for example are clearly eugenic - they are about preventing the consequences of in-breeding," Prof Spencer said.

"We know that because those laws sometimes have exemptions, for example if one of the couple is sterile."