control its outcomes. Such people tend to blame others rather than themselves for their lives' outcomes. It should not be thought, however, that internality is linked exclusively with attribution to effort and externality with attribution to luck (as Weiner's work – see below – makes clear). This has obvious implications for differences between internals and externals in terms of their achievement motivation, suggesting that internal locus is linked with higher levels of need for achievement. Due to their locating control outside themselves, externals tend to feel they have less control over their fate. People with an external locus of control tend to be more stressed and prone to clinical depression. Internals were believed by Rotter (1966) to exhibit two essential characteristics: high achievement motivation and low outer-directedness. This was the basis of the locus-of-control scale proposed by Rotter in 1966, although it was based on Rotter's belief that locus of control is a single construct. Since 1970, Rotter's assumption of uni-dimensionality has been challenged, with Levenson (for example) arguing that different dimensions of locus of control (such as beliefs that events in one's life are self-determined, or organized by powerful others and are chance-based) must be separated. Weiner's early work in the 1970s suggested that orthogonal to the internality-externality dimension, differences should be considered between those who attribute to stable and those who attribute to unstable causes. This new, dimensional theory meant that one could now attribute outcomes to ability (an internal stable cause), effort (an internal unstable cause), task difficulty (an external stable cause) or luck (an external, unstable cause). Although this was how Weiner originally saw these four causes, he has been challenged as to whether people see luck (for example) as an external cause, whether ability is always perceived as stable, and whether effort is always seen as changing. Indeed, in more recent publications (e.g. Weiner, 1980) he uses different terms for these four causes (such as "objective task characteristics" instead of "task difficulty" and "chance" instead of "luck"). Psychologists since Weiner have distinguished between stable and unstable effort, knowing that in some circumstances effort could be seen as a stable cause (especially given the presence of words such as "industrious" in English). Regarding locus of control, there is another type of control that entails a mix among the internal and external types. People that have the combination of the two types of locus of control are