## A Contribution to The Aetiology of Manic-Depressive Insanity (1940)

*Difference in Length of Duration of Earlier and Later Intervals*

It is often stated that during the life of the manic‑depressive the succeeding attacks of illness grow longer, the intervals of health shorter. Little positive evidence, however, has been advanced in support of this view. A contrary observation was made by H.M. Pollock^{1} in an investigation of 878 patients who had had recurrent attacks of manic‑depressive illness. He found that later attacks of illness lasted no longer than earlier ones. On the other hand, there did appear to be some slight increase of duration with the increasing age of the patient. Unfortunately, my material cannot be so analysed as to provide conclusive evidence on this point. I have, however, been able to test for any variation in length of interval between the onsets of earlier and later attacks of illness. The data are analysed in Table 6. One may note in this table a tendency for the first interval to be in general longer than any of the others. This may be in part at least accidentally determined. The data for the earliest attacks, and particularly the first attack, have in my material frequently been obtained from the previous history, i.e. from the history given to the doctor by the patient or his relatives, and are not therefore a matter of documentary record as are those for the later illnesses.

TABLE 6

The degree of statistical significance to be attached to the data of Table 6 may be tested as follows. One calculates the regression of every horizontal row in Table 6 according to the formula:

In this formula x is the number of the column, 1, 2, 3, etc., and y the corresponding mean value of the interval.^{2 }I have carried out this calculation twice over, first with, and secondly without, including the values for the first interval, in order to test whether this first interval significantly influences the result. The standard error of the calculated value of the regression, b, is obtained from the following formula:

The result of these calculation are given in Table 7.

It can be seen from this table that negative values for the horizontal rows are general but by no means universal. A negative value, of course, signifies that the mean length of interval is decreased for later intervals. It can be seen further that in only three rows has this negative value of b any statistical significance, namely in those where b is more than twice as great as its standard deviation. The right half of the table, in which the first interval is left out of consideration, gives some confirmation of the view that these negative values of b have only a very limited significance, for when the first interval is left out of consideration b becomes in some cases positive instead of negative, and in others attains a still lower degree of statistical significance. One may therefore say that there is little evidence of any marked decrease in interval between earlier and between later attacks of illness and that insofar as such a decrease appears to exist it largely concerns the earlier intervals.

In considering what these results may mean a further factor should be taken into consideration. It is very possible that a young person suffering from a fairly severe affective illness will escape admission to clinic or hospital but in later years, suffering from an attack of illness no more severe, will be unable to avoid such an admission because he has already been stamped as a manic‑depressive.

The comparative length of illness in earlier and later attacks is a particularly difficult one to test because of the disturbing factor of the age of the patient at the time of the attack. It is a matter of general clinical experience that affective illnesses occurring at later ages tend to last longer than those of youth or the prime of life, but there is no convincing reason, apart from the not very conclusive evidence provided by Pollock, for believing that this is necessarily true of the recurrent manic‑depressive. Again, observation is likely to be interfered with by extraneous considerations. The well‑known manic‑depressive who has already had. a number of illnesses obtains his discharge from hospital with greater difficulty than a patient who has been admitted for the first time. I have no intention of going into the problem of manic‑depressive end‑states. I would merely wish, on the basis of my material, to throw doubt on the general assumption that a single manic‑depressive attack leaves behind it a disposition to a quicker tempo and longer duration of recurrent illness.

(1) *Amer. J. Psychiat*. (1931) 11 : 567