Psychology papers have a mix of past tense, present tense, future tense, and modal verbs, each used for specific purposes. These guidelines should help you determine when to use which tense in your proposal and thesis. Examples are taken from Cartmill et al. (2014).
Cartmill, E. A., Hunsicker, D., & Goldin-Meadow, S. (2014). Pointing and naming are not redundant: Children use gesture to modify nouns before they modify nouns in speech. Developmental Psychology, 50(6), 1660–1666. https://doi.org/10.1037/a0036003
Use the past tense when referencing the existing findings from specific research. This includes results of studies referenced in your literature review as well as the results of the paper itself.
Valian (1986) observed six English-learning children beginning at age 2 years and found that, by this age, all six were producing determiners with nouns. (Literature Review)
For 15 of the 18 children, the onset of point+noun combinations preceded the onset of determiner+noun combinations by at least one observation session (four months). (Results)
Use the past tense when describing methods for a completed study.
Children were videotaped with their parents at home for 90 minutes every four months from age 14 through 58 months, resulting in 12 sessions per child. (Methods)
Proposals rarely use past tense when referencing the proposed study. One notable exception is if you are using an existing dataset or building on existing, published research. Use past tense to describe anything that was fully completed for another project. If you joined an ongoing project after it began but before any findings were shared, use future tense (more explanation below).
Use present tense to describe generalized claims, like those supported by multiple studies.
Children typically begin the process of learning English by producing nouns (Gentner, 1982; Gleitman, Cassidy, Nappa, Papafragou & Trueswell, 2005). (Introduction)
Use present tense when citing previous work needed to define a concept.
Nominal constituents contain a noun modified by one or several specifiers that function to disambiguate the noun (Huddleston & Pullman, 2002). (Literature Review)
Use present tense for author interpretations, both when citing other authors and when presenting your own interpretations in your Discussion section.
The best evidence [from Valian (1986)] that children are treating determiner + noun combinations as a unit is that they substitute a pronoun for a determiner + noun combination. (Literature Review)
The fact that point+noun combinations did indeed decrease in frequency once determiner+noun combinations emerged in speech provides further support for our hypothesis that using gesture to complement spoken nouns early in development sets the stage for the modification of nouns. (Discussion)
Use simple future tense for anything planned that has not yet happened. Simple future is rarely used in finished theses or papers. All or most of the methods section of your proposal should be written in future tense, since you have not completed the project.
Cartmill (2014) is a completed study, and so does not have examples of future tense. The following is a modification of the example above to show how it might have looked in the project proposal.
Children [will be] videotaped with their parents at home for 90 minutes every four months from age 14 through 58 months, resulting in 12 sessions per child.