Modelo petrofísico a escala de pozo 1 Cálculo del volumen de arcilla (Vsh)

In studies o f the effects o f life events, investigators have seldom described the level o f stressful events that might be expected to be 'normal' in the adolescent population, nor, apparently, regarded the temporal relationship between events and onset o f psychological

disturbance as particularly important. The absence o f control groups in many life events studies underlines these points.

i) The rates of life events in adolescents' lives

Garrison et al (1991b) presented high numbers o f negative events in the previous 12 months (mean for all groups: 10.3). This may be the result o f using checklists with several items which cannot be described as wholly free o f an association with onset o f symptoms. A more rigorous definition o f stressful negative and independent events might have reduced the count. Newcomb et al (1981) reported an average o f nine (rated as positive, negative and neutral) events in a 12 month period in 16-18 year-olds' lives. Using the same questionnaire, Compas et al (1986) give an average o f 7.6 events among 16-19 year olds. Bernstein et al (1989) reported Life Events Checklist rates o f 3.33 for a high anxiety group, and 1.76 for the low anxiety group in a high school sample.

Larson & Ham (1993) reviewed the studies which had looked at rates o f stressful events across late childhood and early adolescence. All the reviewed studies used checklists, and while some studies reported an increase in negative events with age (eg. Garrison et al, 1987 from ages 12-15 years), other studies are cited in which there was a decrease in negative events from 12/14 years to 15/17 years (Compas et al, 1985) and from 16/17 years to 18/19 years (Newcomb et al, 1981). Larson & Ham themselves found an increase with age in negative (but not positive) events (reported by checklist).

Berden et al (1990) also found increasing rates o f negative events with rising age in a large community sample o f Dutch children aged between 6 and 16 years (N=1397). Using a checklist of wholly negative events, the authors found that parents reported their children experiencing a mean o f less than one event over the previous 2 years; 45% of the children had no negative events and only 7.3% experienced three or more negative events over the previous two years. Older children and those from lower socio-economic classes had more events, and there were no differences between boys and girls.

Both Ge et al (1994a) and Wagner & Compas (1990) assessed the number o f stressful events reported by community samples o f teenage girls and boys using a checklist procedure. In each study the girls had more stresshil events, but no figures are given. Several studies have shown that adolescent girls report more stressful events than boys (Burke & Weir, 1978; Compas & Wagner, 1991; Larson & Ham, 1993; Ge et al, 1994a), or report more distress when they have these experiences (Siddique & D'Arcy, 1984; Simmons, Burgeson, Carlton-Ford & Blyth, 1987; Compas & Wagner, 1991; Ge et al, 1994a). This variation between genders o f the rate of stressful events and the variable

response to them have been presented as reasons for age (and gender) differences in rates o f depressive and anxiety disorders in adolescence (Gore & Colten, 1991).

In an associated finding, Brooks-Gunn & Warren (1989) observed that recent stressful events explained significantly more of the variance in depressive symptoms over a 4-year study in early adolescence than did pubertal status (quoted in Ge et al., 1994a: p. 468). However, all o f these studies used life events checklists, with the associated probability o f reporter bias, discussed above.

There appears to be some variation in the rates o f negative events reported by teenagers o f different ethnic background (Gad & Johnson, 1980; Newcomb et al, 1981; Sarason et al, 1978; Garrison et al, 1987), with black teenagers reporting higher rates, but some of these studies have not controlled for social class, which is still highly related to ethnicity. As we have seen, adolescent girls are more likely than boys to report negative events (eg. Garrison et al, 1987).

ii) The timing of stressful events

The timing, as well as the number, o f stressful life events in the year before onset o f symptoms has been found to raise the relative risk o f disturbed mood and behaviour in school-age children (Goodyer et al, 1985, 1987). These authors investigated the effects of life events in a sample attending a psychiatric clinic and matched community controls. Information about the child's life events was obtained fi*om an interview with the mothers. It was found that stressful events tended to cluster in the 16 weeks nearest the

onset o f symptoms, and children who had experienced multiple events were more likely to have had an event within 16 weeks of onset. But events occurred throughout the 12 months and moderately or severely stressful events were found to have long-term, short-term and rapid effects on onset o f psychiatric disorders. Based on this mixed picture the authors suggested that the mechanisms by which the events affect children's psychiatric disorder may vary and be only partially dependent on the number and timing o f the stressful events. There was some support for the concept o f an additive model of the effects of stressful events: the more stressful events the children experienced the more their symptoms. It is worth noting that 30% of clinic cases did not experience any severely stressful events and that 17% of community controls did, indicating that stressful events are not a necessary precursor of all psychiatric disorder in this age-group.

8. Pre-existing psychological status, subsequent events and the onset of depression