David got the paper about Appalachian co-sleeping practices and it has a wonderful review of the literature on co-sleeping. Sharon Abbott mentions a study about Japanese co-sleeping: "Caudill and Plath's (1966) well-known study of Japanese sleeping locations describes their practices throughout the life span, making this an unusual study. They report that the Japanese age of transition away from sleeping with parents is 11, with the process not complete until 15 or 16 years."

The paper is really fascinating. Sharon Abbott is arguing that early family co-sleeping is one of a series of strategies that a parent uses to create a close relationship, an interdependence between parent and child that lasts into adulthood with children continuing to live near their parents. In a paper comparing family characteristics and dynamics in Japan and the U.S. (and so titled) the abstract is this:

This paper uses the Family Environment Scale (FES) to compare and contrast psychosocial environments of Japanese families with those of American families. The aim is to explore some cultural dynamics of each society from the point of view of family behavior. The FES contains 10 subscales, measuring the following family characteristics: Cohesion, Expressiveness, Conflict, Independence, Achievement Orientation, Intellectual-Cultural Orientation, Active-Recreational Orientation, Moral-Religious Emphasis, Organization, and Control. Comparison of each subscale between Japan and the United States revealed that: (a) Cohesion and Control were positively correlated for Japanese parents but negatively correlated for American parents; (b) Achievement Orientation and Intellectual-Cultural Orientation were positively correlated for Japanese parents and uncorrelated for American parents; (c) Cohesion and Control were negatively correlated for American children but uncorrelated for Japanese children; and (d) Achievement and Conflict were positively correlated for Japanese children but uncorrelated for American children. Eight of the 10 constructs were useful for understanding family dynamics in Japan. Independence and Expressiveness, which are important for American families, did not seem to be easily understood concepts for the Japanese. The results and the implications are discussed in terms of cross-cultural understanding between Japan and the United States.

I found it looking for Caudill and Plath's study. I also found a paper (http://archpedi.ama-assn.org/cgi/content/full/153/4/339) that said this:

The 2 cultures endow the child as an organism differently.

In Japan, the infant is seen more as a separate biological organism who from the beginning, in order to develop, needs to be drawn into increasingly interdependent relations with others. In America, the infant is seen more as a dependent biological organism who, in order to develop, needs to be made increasingly independent of others.

Both cultures acknowledge that isolating a child at night is stressful but interpret the experience differently.

In the United States, solitary sleep is thought to engender independence in children and ensure privacy for parents. These values supersede the child's perceived need. United States values of self-reliance and self-assertion lead mothers to encourage more play with objects and more attention to the environment than is seen in Japan. Night-time counterparts may be found in the use of pacifiers and transitional objects to encourage infants to fall asleep alone.

The Japanese acknowledge the same developmental struggle with separation, but, in acceding to the child's need, emphasize the value of dependence as the primary socializing experience. The Japanese word for a common cosleeping arrangement demonstrates this value. "The custom of the child sleeping between the parents is referred to as kawa. Kawa is the Japanese character for a river flowing between 2 banks and kawa is therefore used to refer to the child sleeping between the protective support of the 2 parents." Other cultures or subgroups seem to have a similar emphasis. Thus, the balance between interdependency and autonomy is a useful framework in considering cultural differences in childrearing practices

Anyhow, I don't know if that helps with anything or gives you more ideas to discuss with your mother-in-law. But I found it an interesting perspective on the outcomes of co-sleeping. When David looked at co-sleeping in the northeast of the UK he found that it correlated heavily with other attachment practices like extended breastfeeding and sling carrying. It makes sense if it is seen as a parental strategy (I'm not arguing that it is a strategy that someone is aware of) for bonding children to parents and to create a more interdependent family structure that those things would be associated. It also makes sense that the bonding that occurs in those first few moments of breastfeeding, producing such a huge oxytocin response and bio feedback loop, would produce all of those correlations as well.

Schuyler

Worthman flipped open a book and showed me photographs of big families piled into large, sprawling huts, little kids peeking up from the arms of Mom, older generations wrapped leisurely around the fireplace. “Forager groups are a good place to start, because for much of human history we’ve been occupied with their mode of existence,” she said. “There are the !Kung of Botswana and the Efe of Zaire. For both of these groups, sleep is a very fluid state. They sleep when they feel like it—during the day, in the evening, in the dead of night.”

This, said Worthman, is true of other groups too—the Aché of Paraguay, for example. Late-night sleep, when it happens, is practically a social activity. In addition to procreation, the night is a time of “ritual, sociality, and information exchange.” People crash together in big multigenerational heaps—women with infants, wheezing seniors, domestic animals, chatting hunter buddies stoking the fire—everyone embedded in one big, dynamic, “sensorily rich environment.” This kind of environment is important, said Worthman, because “it provides you with subliminal cues about what is going on, that you are not alone, that you are safe in the social world.”

The more Worthman learned about the communal and interactive nature of non-Western sleep, the more she came to see Western sleep as the strange exception. She laughed again. “It’s funny, because as an anthropologist I’m used to getting weirded out a bit—I mean, you wouldn’t believe the things people do. So after collecting all this material I look at my own bed and go, ‘This is really weird.’” Western sleep, said Worthman, is arid and controlled, with a heavy emphasis on individualism and the “decontextualized person.” Contact is kept to a minimum. The apparent conflict with marriage co-sleeping norms, she notes elsewhere, “has been partially mitigated for Americans by the evolution of bed size from twin, to double, to queen, to king.” She lifted her thin arms and drew a big box in the air. “I mean, think about it—this thing, this bed, is really a gigantic sleep machine. You’ve got a steel frame that comes up from the floor, a bottom mattress that looks totally machinelike, then all these heavily padded surfaces—blankets and pillows and sheets.”

It’s true. Most of us sleep alone in the dark, floating three feet off the ground but also buried under five layers of bedding. I had the sudden image of an armada of solitary humanoids in their big puffy spaceships drifting slowly through the silent and airless immensity of space. “Whoa,” I said.