TOKYO - Revealing one's emotions is frowned upon in Japan. Great value is placed on self-restraint. But sometimes what has long been suppressed comes bursting out with the force of a volcanic eruption. And then there is no stopping it.

An American woman teaching English in Tokyo was recently presented with an extraordinarily candid confession by one of her male students. She thinks it was precisely because she was a foreigner that the man chose to open up to her.

The student, in his early twenties, was receiving private English conversation tuition from the American woman who in the course of one lesson asked him about his mother. The student fell silent, looked out of the window, and suddenly burst into tears.

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After some time he calmed down, and, with little further prompting, began to talk about an experience eight years before. It was during his final year at school, when he was immersed in the 'examination hell' that demands long hours of study, cram schools and demonic dedication from those who want to get into a good university. One day he was offered, and received, sexual favours from his mother to encourage him to persevere with his schoolwork. Afterwards he felt crushing guilt, although he came to suspect that something similar had happened to at least one of his classmates. But he had never before spoken directly about it.

Satoru Saito, head of the sociopathology department at the Psychiatric Research Institute of Tokyo, doubts that mother-son incest is any more common in Japan than elsewhere. But, he says, 'emotional incest' between mothers and their sons is almost a defining feature of Japanese society - 'the entire culture has this undertone'.

It is a vicious circle, says Dr Saito. Women who rarely see their husbands except late at night when they come home after drinking with workmates automatically funnel their emotions towards their children, particularly towards their sons.

The sons grow up with over-indulgent mothers who smother them in affection, preventing them from developing emotional independence. The sons accept the power their mothers have over them, and, when they begin dating, they naturally look for another woman to mother them.

This maza-con (mother complex) is a ritual complaint of young Japanese women seeking a husband. But as soon as they get married, they inevitably transfer the emotions that their husbands will not absorb to their children. And so the cycle perpetuates itself.

'The Japanese male does not like responsibility,' says Dr Saito. 'He is always controlled by someone else: his mother, his teacher, his boss in the company. I call this the ofukuro (pouch) function - he seeks to be protected in a maternal pouch all the time. We call our school the 'mother school'; employees regard their company as some kind of mother figure.'

The mother complex even extends to the state, whose organs of control - police, tax inspectors, railway guards - treat citizens like precious children, in constant need of surveillance and gentle chiding.

'A train is coming. It is dangerous, so please stand behind the white line on the platform,' is the recorded message that is played every time a train comes into a subway station in Tokyo.

It continues after work. Men who do not want to face up to the responsibility of the father/husband role will delay going home by taking refuge in a hostess bar, where they find the mama-san (bar-mother) and her female cohorts who treat their guests as overgrown schoolboys, pouring their drinks, lighting their cigarettes and listening to their complaints about work and, with mock disapproval, to their dirty jokes.

All this goes on in a cosy, womb-like room with low couches, soft music and a pervasive undertone of sexual promise.

Dr Saito's institute runs a telephone hotline for people with mental problems. One man calls regularly to tell of incestuous relations with his mother many years ago which he claims still haunt him. Dr Saito is almost certain the man is reflecting his fantasies, rather than recalling real events in the past, since over time the stories contradict one other in small details. But, real or imagined, the incest theme is there.

'There is no clear distinction between male-female relations and mother- son relations,' says Dr Saito. 'Japanese males are always mixing these two: they want to assert their sexuality, but at the same time they want to be held by their mothers - warm, safe, secure.'