Eugenics

There is a small section of humanity who might be advised or perhaps even be prevented from entering into marriage (to be defined later by the House of Justice)

"The Bahá'í Teachings do not only encourage marital life, considering it the natural and normal way of existence for every sane, healthy and socially-conscious and responsible person, but raise marriage to the status of a divine institution, its chief and sacred purpose being the perpetuation of the human race--which is the very flower of the entire creation--and its elevation to the true station destined for it by God.

"That there should be, however, certain individuals who by reason of some serious deficiency, physical or mental, would be incapacitated to contract marriage and enjoy the blessings of an enduring and successful marital life is only too evident, but these constitute only a very small section of humanity, and are therefore merely an exception, and their condition cannot possibly invalidate what an all-wise and loving Providence has decreed to be the normal way to a fruitful and constructive social existence.

"The exact conditions and circumstances under which such incapacitated individuals should be advised or even prevented perhaps from entering into any sort of marital existence have not been specified in the Bahá'í Writings, but will have to be defined later on by the Universal House of Justice. In the meantime, those believers who consider themselves as falling into the above category would do well, before taking any final decision themselves, to consult medical experts, who are both conscientious and competent, and to abide by their recommendation.

"This is what the Guardian would advise you to do, and he will pray that you may be guided in reaching the right decision in this assuredly delicate and indeed most vital matter confronting you at present. Whether your illness is the result of any inherent constitutional weakness and inherited predisposition is a question which you should refer to experts in the medical field, though even expert physicians themselves may in very few cases find it exceedingly hard, if not altogether impossible, to give a final and decisive answer."

(From a letter written on behalf of Shoghi Effendi to an individual believer, April 15, 1939, in Lights of Guidance, no. 1262)