It's like Webster says. The practice of marriage within a social group. Examples of such a group include occupation, family, colour, caste, nationality and religion. Religion is probably the most relevant grouping worldwide. The Amish community and Orthodox Jews may be the most famous practitioners but endogamy, to varying degrees, is extremely common.

Endogamy encourages group cohesion and the continuity of group cultural practices. In some contexts it may have important socio-economic underpinnings. In agrarian or pastoral societies for example, marriage within a group ensures group ownership of land or animals over generations. Indeed in some societies, exogamy (i.e. marriage outside the group) is strongly discouraged and may be punishable - by social exclusion, violence or otherwise.

In small groups, the practice of endogamy often (but not necessarily) entails consanguineous marriage. This is usually defined as marriage between two second cousins or closer relations. Genetic inbreeding has the well-known unfortunate consequence of concentrating recessive alleles. Groups practising strict endogamy therefore commonly suffer a high burden of inherited disease. Mother Nature it seems, doesn't approve.

Perhaps you are bored. Let us sing.

The Endogamy Song (Let's Keep The Good Genes In)

Scene : A Monastery garden. A Monk is carefully examining a row of bean plants. Enter Parents and Errant Son, deep in discussion. Fingers and tongues are wagging. Gradually, the Parents begin to sing and the Beans begin to dance.

Well there once was a Monk named something or another

While growing peas he was amazed to discover

That the traits of the beans

Were transmitt-ed in genes

Let's keep the good genes in!

As with beans well so with people -

Genes they are our stalk and steeple

Your father's nose and your mother's wits

And your grandma's pains and your uncle's fits

They're all right there in the Book o' Doom

That is writ by the Maker in your Momma's womb!

Let's keep the good genes in!

Our kids will have our Re-li-gious Culture

Our tribe will not be left to vultures

It's our ge-ne-tic pick `n mix

Not your fre-ne-tic teen antics {The Monk frowns suspiciously}

That will decide who marries who!

Let's keep the good memes in!

The priests and the laity are quite agreed

That the trees we plant must be of noble seed

Our councils ecumenical

Preach the crypto-eugenical:

Let's keep the good genes in!

If you run off and marry some tart

You'll dilute our stock and you'll break our hearts

Mix Robinson's and Ri-be-na?! {The Monk shakes his head sadly}

Mix Lion and Hy-e-na?! {The beans look appalled and wail}

Let's keep the good genes in!

We'll find you a nice girl, someone you like

Would you look at this one here, boy, what a sight!

She got hips that launched a thousand ships

Of hopeful in-law's fishing trips

She's a respectable bespectacled genetical receptacle

She'll keep a tidy home!

She prays, she cooks every del-ect-able

You got no cause to moan!

Let's keep the good genes in!

Well Mummy and Da-ddy I see what ya mean

About the birds and the bees and the peas and the genes

If it's really so imperative

I'll find a foxy relative...

We'll keep the good genes in!

{The Beans, the Parents, the Monk and the Errant Son join hands and dance joyfully into a whirligig. Camera fades.}

• Modell, B and Darr, A. Genetic Counselling and Customary Consanguineous Marriage. Nature Reviews Genetics 3, 225-229 (March 2002)