UCD CENTRE FOR ECONOMIC RESEARCH WORKING PAPER SERIES 2008 Handedness and Depression: Evidence from a Large Population Survey

This paper uses a new large population survey from twelve European countries to measure the association between handedness and depression. It is found that that depressive symptoms are significantly higher amongst lefthanded men. While 19% of right handed men report experiencing depressive symptoms for at least a two week period, the figure for left handed men is almost 25%. For women the corresponding percentages are 33% and 36% respectively but the difference is not statistically significant. Using the EURO-D depression scale gives equivalent results. These results are consistent with one finding from an existing small scale study. * [email protected] tel: 353 1 716 4632. fax: 353 1 716 1108. Address: Geary Institute, University College Dublin, Belfield, Dublin 4, Ireland. Introduction A number of papers consider the relationship between handedness and various forms of psychiatric morbidity including depression and schizophrenia. There is no consensus about the association between handedness and major depression and no strong theoretical argument about what any such relationship might be – unlike the case of schizophrenia or manic depression (see Klar (1999), Crow (1997) or Sommer et al (2001) for example). It is striking that virtually all the existing studies are from small clinical samples or somewhat larger, but still small, datasets, based on students. Hence it is difficult to infer anything about the relationship in the general population. In particular if the effect sizes of interest are small then the usual tests may be seriously underpowered. Hence it is essential to examine this relationship in larger, more representative datasets. Biederman et al (1994) find a positive association between left-handedness and depression in a sample of 260 boys between the age of 6 and 17. Bruder et al (1981), who also find a positive effect, analyse a sample of 48. Amongst papers which find a negative association are Merrin (1984) using a sample of 52 individuals with schizophrenia and Muscovitch, Strauss and Olds (1981) using a sample of patients with unipolar endogenous depression. Overby (1994) and Elias et al (2001) are two nonclinical studies using samples of university students. Interestingly, they find opposite results. The former paper finds a positive association for females only and no effect for men whereas the latter finds a positive effect for men only and no effect for women. This note utilises a new large cross-country dataset, SHARE, to revisit the question. 1 See Coren (1993), Gelman & Weekliem (2007) for example 2 See Elias et al (2001) for a review of the findings. 1 Data and Methods The dataset used is SHARE: the Survey of Ageing, Health and Retirement in Europe. This collects data from nationally representative samples of the non-institutional population aged 50 years and older. The primary sampling unit is a household and all individuals in the household who are in the target age category are interviewed. This paper used release 2 of the dataset which includes 12 countries. The sampling plan follows a complex probabilistic multistage design. Hence probability weights (calibrated by age and sex) are used to ensure that the sample is representative. For estimation purposes, each country is treated as a stratum and allows for clustering within households. The size of the sample used in the estimation is 27,482. Two measures of mental health are used. Euro-D is a 12 item scale developed by the EURODEP Consortium (Prince et al 1999, Copeland 1999). The items are a subset of those on the Geriatric Mental State. The scale was created to provide a simple measure of the extent of depressive symptoms that could be used for comparing across European countries. The questions refer to the presence of these symptoms in the last month. The second measure was a binary variable indicating the yes/no response to the question “Has there been a time or times in your life when you suffered from symptoms of depression which lasted at least two weeks?” Handedness was measured simply by asking the respondents what their dominant hand was. Descriptive statistics for the data are in Table 1. The proportion of left-handers is about 7%, somewhat lower than many population estimates. However given that the average age is 64 and that the observed frequency of left-handedness is much lower amongst older populations this is not exceptionally low. 3 They are Austria, Belgium, Denmark, France, Germany, Greece, Italy, Netherlands, Spain, Sweden, Switzerland and Israel. The first release of the data which was collected in 2004 consists of the first 11 countries listed. Preliminary data from Israel, collected in 2005-2006, was included in release 2. Visit http://www.share-project.org/ for more information. 4 See McManus (2002) Figure 9.1. 2 To see how the mean of Euro-D varied with handedness we use least squares regression with handedness interacted with sex as explanatory variables. The comparison group is right-handed males. Since the other outcome of interest is binary we estimate a probit model which predicts the probability that an individual will have ever experienced depressive symptoms using the same explanatory variables as the first model. One could estimate a much richer model with additional covariates that will predict depression (including marital status, age, history of chronic illness for example). However including these in the model – while they may be of interest in their own righthas no effect on the results of interest here. This is not surprising since these variables are largely orthogonal to handedness hence there is no omitted variable bias. Results The results of the data analysis are in Table 2. The first column reports the coefficients from the regression of the Euro-D scale on the sex by handedness interaction. Lefthanded males have a higher level of depressive symptoms than right-handed males being 0.227 higher on average. Since the mean for the comparison group is 1.814, this is about a 12.5% difference. Another way of looking at this is that the difference is about one quarter of the difference due to sex, the mean Euro-D score for females being higher than males by approximately 1. For females, the mean level of depressive symptoms is about 0.05 lower for left-handed females compared to right-handed females. However this difference, aside from being small in absolute terms, is not statistically significant (p value = 0.625). The second column reports the marginal effects from the probit model. The results indicate that left-handed males are 5.5% (i.e. 5.5 percentage points) more likely to have ever experienced depression compared to right handed males. For the comparison group the baseline incidence is about 19 percentage points so left-handed males are