Laughter is caused by the slackening or relaxation of the nerves. It is an ideal condition and not physical. Laughter is the visible effect of an invisible cause. For example, happiness and misery are super-sensuous phenomena. One cannot hear them with the ears or touch them with his hands. Happiness is a spiritual state.

‘Abdu’l-Bahá, Star of the West, Volume 7, p. 102.

Abandonment of "a frivolous conduct" does not imply that a Bahá’í must be sour-faced or perpetually solemn. Humour, happiness, joy are characteristics of a true Bahá’í life. Frivolity palls and eventually leads to boredom and emptiness, but true happiness and joy and humour that are parts of a balanced life that includes serious thought, compassion and humble servitude to God are characteristics that enrich life and add to its radiance.

The Universal House of Justice, 1997 Jan 12, The Humourist, p. 2-3 an essential element our comprehension of reality.

Universal House of Justice, 23 July 1985 to an individual

The Universal House of Justice has requested us to acknowledge your letter of November 6, 1977 and to say that while laughter should not be suppressed or frowned upon, it should not be indulged in at the expense of the feelings of others. What one says or does in a humorous vein should not give rise to prejudice of any kind. You may recall ‘Abdu’l-Bahá's caution "Beware lest ye offend the feelings of anyone, or sadden the heart of any person..."

The Universal House of Justice, 1997 Jan 12, The Humourist, p. 2

‘Abdu’l-Bahá loved laughter and His laughter was often a source of solace . . . He said that . . . It is good to laugh. Laughter is spiritual relaxation. When they were in prison, He said, and under the utmost deprivation and difficulties, each of them at the close of the day would relate the most ludicrous event which had happened. Sometimes it was difficult to find one but always they would laugh until the tears would run down their cheeks. Happiness, He said, is never dependent upon material surroundings, otherwise, how sad those years would have been. As it was they were always in the utmost state of joy and happiness . . . “My home is the home of laughter and mirth”, He declared and indeed it was so. He delighted in gathering together people of various races, colours, nations, and religions in unity and cordial friendship . . . As He said on another occasion, “My home is the home of peace. My home is the home of joy and delight. My home is the home of laughter and exultation. Whoever enters through the portals of this home must go out with gladsome heart."

Annamarie Honnold, Vignettes from the Life of ‘Abdu’l-Bahá, pp.149-151